tools headers: Add missing perf_event.h include

We need perf_event.h include for 'struct perf_event_mmap_page'.

Link: http://lkml.kernel.org/n/tip-bolqkmqajexhccjb0ib0an8w@git.kernel.orgSigned-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190822111141.25823-2-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/include/linux/ring_buffer.h

@@ -2,6 +2,7 @@
 #define _TOOLS_LINUX_RING_BUFFER_H_
 
 #include <asm/barrier.h>
+#include <linux/perf_event.h>
 
 /*
  * Contract with kernel for walking the perf ring buffer from