perf tools: Move two variables usied in libperf from perf.c

The use_browser and perf_version_string variables are both declared in
perf.c but they are also referenced by other functions of libperf.a.

Therefore a user linking an own main() with libperf.a must declare those
two variables in their files even if the files never use the browser or
the version information.

This patch fixes this issue by moving use_browser and
perf_version_string out of perf.c to some other files.

Signed-off-by: Soramichi Akiyama <akiyama@m.soramichi.jp>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170117002237.c1aec0ce3b4d675dca018deb@m.soramichi.jp
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/header.c

@@ -41,6 +41,8 @@ static const u64 __perf_magic2_sw = 0x50455246494c4532ULL;
 
 #define PERF_MAGIC	__perf_magic2
 
+const char perf_version_string[] = PERF_VERSION;
+
 struct perf_file_attr {
 	struct perf_event_attr	attr;
 	struct perf_file_section	ids;