perf tools: Move install-gtk target into rules area

perf tools: Move install-gtk target into rules area

The upcoming fixdep fix needs all targets at the same area, so they'll
fit under a signal condition block.

Move install-gtk target into the rules section.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1480884178-8072-4-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>