ps: include error.h

This code currently uses error_at_line() from error.h, so pull it in.
Long term, this might get moved to c.h as a local helper on err.h,
but I have no idea.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

ps/global.c

@@ -17,6 +17,7 @@
 #include <string.h>
 #include <termios.h>
 #include <unistd.h>
+#include <error.h>
 
 #include <sys/ioctl.h>
 #include <sys/stat.h>