perf: clear out make flags when calling kernel make kernelver

When generating the perf version from the kernel version using 'make
kernelver' it is necessary to clear out any MAKEFLAGS otherwise they may
trigger additional output which pollute the contents.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>