selftests: Add headers target

Some selftests need access to a full UAPI headers tree, for example when
building with nolibc which heavily relies on UAPI headers.
A reference to such a tree is available in the KHDR_INCLUDES variable,
but there is currently no way to populate such a tree automatically.

Provide a target that the tests can depend on to get access to usable
UAPI headers.

Signed-off-by: Thomas Weißschuh <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Vincenzo Frascino <[email protected]>
Acked-by: Shuah Khan <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

tools/testing/selftests/lib.mk

@@ -228,4 +228,7 @@ $(OUTPUT)/%:%.S
 	$(LINK.S) $^ $(LDLIBS) -o $@
 endif
 
-.PHONY: run_tests all clean install emit_tests gen_mods_dir clean_mods_dir
+headers:
+	$(Q)$(MAKE) -C $(top_srcdir) headers
+
+.PHONY: run_tests all clean install emit_tests gen_mods_dir clean_mods_dir headers