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 <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/all/20250226-parse_vdso-nolibc-v2-8-28e14e031ed8@linutronix.de

Author: t-8ch

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