selftests/powerpc: Fix the clean rule since recent changes

The clean rule is broken for the powerpc tests:

  make[1]: Entering directory 'tools/testing/selftests/powerpc'
  Makefile:63: warning: overriding recipe for target 'clean'
  ../lib.mk:51: warning: ignoring old recipe for target 'clean'
  /bin/sh: 3: Syntax error: end of file unexpected (expecting "done")
  Makefile:63: recipe for target 'clean' failed

Fixes: a8ba798b

 ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>