perf gtk: Only support --gtk if compiled in

perf gtk: Only support --gtk if compiled in

If HAVE_GTK2_SUPPORT isn't defined then --gtk can't succeed, don't
support it as a command line option in this case.

v2. Is a rebase. Patch appears to have been missed in:
https://lore.kernel.org/lkml/Ygu40djM1MqAfkcF@kernel.org/

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: xaizek <xaizek@posteo.net>
Link: https://lore.kernel.org/r/20220707203836.345918-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>