selftests: x86: override clean in lib.mk to fix warnings

Add override with EXTRA_CLEAN for lib.mk clean to fix the following
warnings from clean target run.

Makefile:44: warning: overriding recipe for target 'clean'
../lib.mk:55: warning: ignoring old recipe for target 'clean'

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>