selftests/powerpc/switch_endian: Fix out-of-tree build

For the out-of-tree build to work we need to tell switch_endian_test
to look for check-reversed.S in $(OUTPUT).
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/switch_endian/Makefile

@@ -8,6 +8,7 @@ EXTRA_CLEAN = $(OUTPUT)/*.o $(OUTPUT)/check-reversed.S
 top_srcdir = ../../../../..
 include ../../lib.mk
 
+$(OUTPUT)/switch_endian_test: ASFLAGS += -I $(OUTPUT)
 $(OUTPUT)/switch_endian_test: $(OUTPUT)/check-reversed.S
 
 $(OUTPUT)/check-reversed.o: $(OUTPUT)/check.o