selftests/powerpc: Install tempfile so the subpage_prot_file test works

We forgot to install the tempfile, so when the selftests are installed
and then run the subpage_prot_file test fails.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/mm/Makefile

@@ -2,8 +2,9 @@ noarg:
 	$(MAKE) -C ../
 
 TEST_PROGS := hugetlb_vs_thp_test subpage_prot
+TEST_FILES := tempfile
 
-all: $(TEST_PROGS) tempfile
+all: $(TEST_PROGS) $(TEST_FILES)
 
 $(TEST_PROGS): ../harness.c