Provide strlcpy declaration

strlcpy(3) might not be visible since it is declared in <bsd/string.h>.
This can lead to warnings, like:

    fields.c: In function 'change_field':
    fields.c:103:17: warning: implicit declaration of function 'strlcpy'; did you mean 'strncpy'? [-Wimplicit-function-declaration]
      103 |                 strlcpy (buf, cp, maxsize);
          |                 ^~~~~~~
          |                 strncpy

    ../lib/fields.c:103:17: warning: type of 'strlcpy' does not match original declaration [-Wlto-type-mismatch]
      103 |                 strlcpy (buf, cp, maxsize);
          |                 ^
    /usr/include/bsd/string.h:44:8: note: return value type mismatch
       44 | size_t strlcpy(char *dst, const char *src, size_t siz);
          |        ^
    /usr/include/bsd/string.h:44:8: note: type 'size_t' should match type 'int'
    /usr/include/bsd/string.h:44:8: note: 'strlcpy' was previously declared here
    /usr/include/bsd/string.h:44:8: note: code may be misoptimized unless '-fno-strict-aliasing' is used

lib/Makefile.am

@@ -11,6 +11,7 @@ libshadow_la_CPPFLAGS += -DVENDORDIR=\"$(VENDORDIR)\"
 endif
 
 libshadow_la_CPPFLAGS += -I$(top_srcdir)
+libshadow_la_CFLAGS = $(LIBBSD_CFLAGS)
 
 libshadow_la_SOURCES = \
 	commonio.c \

src/Makefile.am

@@ -13,6 +13,8 @@ AM_CPPFLAGS = \
 	-I$(top_srcdir) \
 	-DLOCALEDIR=\"$(datadir)/locale\"
 
+AM_CFLAGS = $(LIBBSD_CFLAGS)
+
 # XXX why are login and su in /bin anyway (other than for
 # historical reasons)?
 #