package/perf: fix build failures with high 'make -j' values

The 'perf' Makefile is flaky on some kernel versions. It will still parallelize
the build even with 'make -j1' because it explicitly invokes a sub-make with
the correct flags. But the top-level make is not smart enough to handle large
'-j' values.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>