fixup! WIP: add header test target copied from i915

Fixes out of tree header check after Fixes: b1992c3 ("kbuild: use
$(src) instead of $(srctree)/$(src) for source directory").

Signed-off-by: Janne Grunau <j@jannau.net>

Author: jannau

drivers/gpu/drm/apple/Makefile

@@ -23,7 +23,7 @@ no-header-test := \
 
 always-y += \
 	$(patsubst %.h,%.hdrtest, $(filter-out $(no-header-test), \
-		$(shell cd $(srctree)/$(src) && find * -name '*.h')))
+		$(shell cd $(src) && find * -name '*.h')))
 
 quiet_cmd_hdrtest = HDRTEST $(patsubst %.hdrtest,%.h,$@)
       cmd_hdrtest = $(CC) $(filter-out $(CFLAGS_GCOV), $(c_flags)) -S -o /dev/null -x c /dev/null -include $<; touch $@