programs/util.c: fix build without st_mtime

programs/util.c: fix build without st_mtime

Since version 1.4.5 and commit
5af8cb7aea8d890b4801e50e5274371510f2cf33, 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>