perf/core: Fix !CONFIG_PERF_EVENTS build warnings and failures

perf/core: Fix !CONFIG_PERF_EVENTS build warnings and failures

sparc64 runs into this warning:

  include/linux/security.h:1913:52: warning: 'struct perf_event' declared inside parameter list will not be visible outside of this definition or declaration

which is escalated to a build error in some of the .c files due to -Werror.

Fix it via a forward declaration, like we do for perf_event_attr, the stub inlines
don't actually need to know the structure of this struct.

Fixes: da97e18458fb: ("perf_event: Add support for LSM and SELinux checks")
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>