perf ui gtk: Move gtk .so name to the only place where it is used

No need to pollute util.h with this.

Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-kec0chbdtgrd71o3oi2kz2zt@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>