src: move _GNU_SOURCE definition before all includes in files that need vasprintf()

Build of input-libinput fails on GCC-11.2 although succeeds on GCC-14.2.
Function vasprintf() becomes implicitle declared because its definition
in stdio.h is guarded by __GLIBC_USE_LIB_EXT2 which depends on _USE_GNU,
which in turn depends on __GNU_SOURCE. __GNU_SOURCE is defined at
the beginning of util_string.h before #include <stdio.h>, and this file
is included by util_strings.c and xf86libinput.c. The latter file
includes first a lot of headers in /usr/include and /usr/include/xorg,
and finally util_string.h. If some of the previous header files
#include <stdio.h>, then it is included first without _GNU_SOURCE, and
vasprintf() is left undefined, which causes a build error.

This patch moves #define _GNU_SOURCE to util_strings.c and xf86libinput.c
from util_strings.h, thus making vasprintf() declaration safe and
independent of a specific compiler.

Signed-off-by: Oleh Nykyforchyn <olen.nyk@gmail.com>

src/util-strings.c

@@ -23,6 +23,7 @@
  */
 
 #include "config.h"
+#define _GNU_SOURCE
 
 #include "util-strings.h"
 

src/util-strings.h

@@ -24,9 +24,6 @@
 
 #pragma once
 
-#include "config.h"
-#define _GNU_SOURCE
-
 #include <assert.h>
 #include <ctype.h>
 #include <errno.h>

src/xf86libinput.c

@@ -24,6 +24,7 @@
  */
 
 #include "config.h"
+#define _GNU_SOURCE
 
 #include <errno.h>
 #include <fcntl.h>