perf tools: Remove '?=' Makefile STRIP assignment

No need to use '?=' assignment for STRIP variable, the standard
'=' does the same job without creating confusion.

Suggested-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Stephane Eranian <eranian@google.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: http://lkml.kernel.org/r/1369398928-9809-25-git-send-email-jolsa@redhat.com


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