Disable use of undefined DEF_WEAK macro in strptime.c

The strptime.c code invokes, what is presumably an undefined macro
called, DEF_WEAK. The compiler can't resolve this to a function, so
being traditional C, it invents a function taking arbitrary args and
returning an int.

GCC 9.5.0 used in the reproducible build warns about this problem
but sdk/Makefile.source is throwing stderr from the tlibc build
by sending it all to /dev/null, so the warning is never seen.

New GCC 14 has turned this warning into a fatal error, such that
this bug breaks the build outside of the NixOS envirnoment.

The other use of DEF_WEAK in tlibc/time/localtime.c was surrounded
by '#if 0', so do the same in strptime.c

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

sdk/tlibc/time/strptime.c

@@ -89,7 +89,9 @@ strptime(const char *buf, const char *fmt, struct tm *tm)
 {
 	return(_strptime(buf, fmt, tm, 1));
 }
+#if 0
 DEF_WEAK(strptime);
+#endif
 
 static char *
 _strptime(const char *buf, const char *fmt, struct tm *tm, int initialize)