programs/util.c: fix build without st_mtime

Since version 1.4.5 and commit
5af8cb7aea, if st_mtime is not defined,
programs/util.c uses utime without including utime.h which will raise
the following build failure on some of the buildroot autobuilders:

util.c: In function 'UTIL_setFileStat':
util.c:161:24: error: storage size of 'timebuf' isn't known
         struct utimbuf timebuf;
                        ^~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/be902c5d110f37bce622a2215191f155b7d3e7e0

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

programs/util.c

@@ -28,7 +28,7 @@ extern "C" {
 #  include <io.h>         /* _chmod */
 #else
 #  include <unistd.h>     /* chown, stat */
-#  if PLATFORM_POSIX_VERSION < 200809L
+#  if PLATFORM_POSIX_VERSION < 200809L || !defined(st_mtime)
 #    include <utime.h>    /* utime */
 #  else
 #    include <fcntl.h>    /* AT_FDCWD */