handle clash with include/linux/perf_event.h, add C++ support
In case include/perfmon/perf_event.h is included in a module
which also includes the official perf_event.h kernel header file
we need to ensure there is some sort of mutual exclusion.
Always include <linux/perf_event.h> first. Otherwise, we
cannot solve the variable namespace clashes.
Also add C++ support to perf_event.h
Signed-off-by: Stephane Eranian <email@example.com>