perf: fix compile error when rebuilding

Fix error like:
Makefile.perf:597: recipe for target
'intel_x86_64-wrs-linux/perf/1.0-r9/perf-1.0/perf-in.o] Error 2
Makefile.perf:220: recipe for target 'sub-make' failed
make[1]: *** [sub-make] Error 2
Makefile:69: recipe for target 'all' failed
make: *** [all] Error 2

during rebuilding, when kernel version changed, some
source files also maybe changed. It could happened
that the source under ${S} messed up with some already
dropped files and also new files. Fix by clean the ${S}
before copy source files.

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>