perf: Add support for Linux 3.13

The Makefile has been renamed to Makefile.perf; so now we check both
Makefile and Makefile.perf to support 3.7 - 3.12 and 3.13 - beyond.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>