kfreeBSD: include config.h for _GNU_SOURCE

strcasestr(3) is a GNU extension and when compiling freebsd/Platform.c
on kfreebsd for Debian <string.h> is included before we define
_GNU_SOURCE, so the function is not available.

In file included from ./Object.h:16,
                 from ./ListItem.h:12,
                 from ./Meter.h:16,
                 from ./Header.h:10,
                 from ./Action.h:15,
                 from freebsd/Platform.h:13,
                 from freebsd/Platform.c:8:
./XUtils.h: In function ‘String_contains_i’:
./XUtils.h:43:11: warning: implicit declaration of function ‘strcasestr’; did you mean ‘strcasecmp’? [-Wimplicit-function-declaration]
   43 |    return strcasestr(s1, s2) != NULL;
      |           ^~~~~~~~~~
      |           strcasecmp
./XUtils.h:43:30: warning: comparison between pointer and integer
   43 |    return strcasestr(s1, s2) != NULL;
      |                              ^~
In file included from ./Object.h:16,
                 from ./ProcessList.h:16,
                 from freebsd/FreeBSDProcessList.h:15,
                 from freebsd/FreeBSDProcessList.c:8:
./XUtils.h: In function ‘String_contains_i’:
./XUtils.h:43:11: warning: implicit declaration of function ‘strcasestr’; did you mean ‘strcasecmp’? [-Wimplicit-function-declaration]
   43 |    return strcasestr(s1, s2) != NULL;
      |           ^~~~~~~~~~
      |           strcasecmp
./XUtils.h:43:30: warning: comparison between pointer and integer
   43 |    return strcasestr(s1, s2) != NULL;
      |                              ^~

freebsd/Platform.c

@@ -5,6 +5,8 @@ Released under the GNU GPLv2, see the COPYING file
 in the source distribution for its full text.
 */
 
+#include "config.h" // IWYU pragma: keep
+
 #include "Platform.h"
 
 #include <devstat.h>