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

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

Makefile:36: 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>