selftests/powerpc: Fix the pmu install rule

My patch to add install support for the powerpc selftests had a typo,
leading to the three tests in the pmu directory itself not being
installed.

Fixes: 6faeeea4 ("selftests: Add install support for the powerpc tests")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/pmu/Makefile

@@ -26,7 +26,7 @@ override define EMIT_TESTS
 	$(MAKE) -s -C ebb emit_tests
 endef
 
-DEFAULT_INSTALL := $(INSTALL_RULE)
+DEFAULT_INSTALL_RULE := $(INSTALL_RULE)
 override define INSTALL_RULE
 	$(DEFAULT_INSTALL_RULE)
 	$(MAKE) -C ebb install