perf check-headers.sh: Add support to check 2 independent files

Add 'check_2' function to check 2 different files, the 'check' function
stays to check files that differs only in the prefix path.

In upcoming changes we need to check header files in locations which
don't follow the prefix logic.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180423090823.32309-3-jolsa@kernel.org


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