perf tools: Fix tags/TAGS targets rebuilding

Once the tags/TAGS file is generated it's never rebuilt until it's
removed by hand.

The reason is that the Makefile does not treat tags/TAGS as targets but
as files and thus won't rebuilt them once they are in place.

Adding PHONY tags/TAGS targets into Makefile.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20131126125412.GJ1267@krava.brq.redhat.com


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