fix compilation of fetch_cb on gcc 7

Original message:

```
fetch_cb.c:80:29: error: ‘h’ directive output may be truncated writing 1 byte into a region of size between 0 and 14 [-Werror=format-truncation=]
   snprintf(str, sizeof str, "%02ldh%02ldm",
                             ^~~~~~~~~~~~~~
fetch_cb.c:80:29: note: directive argument in the range [0, 59]
fetch_cb.c:80:3: note: ‘snprintf’ output between 7 and 21 bytes into a destination of size 16
   snprintf(str, sizeof str, "%02ldh%02ldm",
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       eta / 3600, (eta % 3600) / 60);
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fetch_cb.c:83:30: error: ‘%02ld’ directive output may be truncated writing between 2 and 19 bytes into a region of size 16 [-Werror=format-truncation=]
   snprintf(str, sizeof str, "%02ldm%02lds",
                              ^~~~~
fetch_cb.c:83:29: note: directive argument in the range [-153722867280912930, 60]
   snprintf(str, sizeof str, "%02ldm%02lds",
                             ^~~~~~~~~~~~~~
fetch_cb.c:83:29: note: directive argument in the range [-59, 59]
fetch_cb.c:83:3: note: ‘snprintf’ output between 7 and 25 bytes into a destination of size 16
   snprintf(str, sizeof str, "%02ldm%02lds",
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       eta / 60, eta % 60);
       ~~~~~~~~~~~~~~~~~~~
fetch_cb.c:80:29: error: ‘h’ directive output may be truncated writing 1 byte into a region of size between 0 and 14 [-Werror=format-truncation=]
   snprintf(str, sizeof str, "%02ldh%02ldm",
                             ^~~~~~~~~~~~~~
fetch_cb.c:80:29: note: directive argument in the range [0, 59]
fetch_cb.c:80:3: note: ‘snprintf’ output between 7 and 21 bytes into a destination of size 16
   snprintf(str, sizeof str, "%02ldh%02ldm",
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       eta / 3600, (eta % 3600) / 60);
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fetch_cb.c:83:30: error: ‘%02ld’ directive output may be truncated writing between 2 and 19 bytes into a region of size 16 [-Werror=format-truncation=]
   snprintf(str, sizeof str, "%02ldm%02lds",
                              ^~~~~
fetch_cb.c:83:29: note: directive argument in the range [-153722867280912930, 60]
   snprintf(str, sizeof str, "%02ldm%02lds",
                             ^~~~~~~~~~~~~~
fetch_cb.c:83:29: note: directive argument in the range [-59, 59]
fetch_cb.c:83:3: note: ‘snprintf’ output between 7 and 25 bytes into a destination of size 16
   snprintf(str, sizeof str, "%02ldm%02lds",
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       eta / 60, eta % 60);
       ~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
```

bin/xbps-install/fetch_cb.c

@@ -65,7 +65,7 @@ static const char *
 stat_eta(const struct xbps_fetch_cb_data *xfpd, void *cbdata)
 {
 	struct xferstat *xfer = cbdata;
-	static char str[16];
+	static char str[25];
 	long elapsed, eta;
 	off_t received, expected;