tools: always create $STAGING_DIR/usr/{include,lib}

rules.mk always passes these as -I/-L to the toolchain.

Fixes rare errors like:
cc1: error: staging_dir/target-aarch64_cortex-a53_musl/usr/include: No such file or directory [-Werror=missing-include-dirs]

Signed-off-by: Andre Heider <a.heider@gmail.com>
Acked-by: Paul Spooren <mail@aparcar.org>
Acked-by: Rosen Penev <rosenp@gmail.com>

tools/Makefile

@@ -118,7 +118,7 @@ define PrepareStaging
 		$(if $(QUIET),,set -x;) \
 		mkdir -p "$$dir"; \
 		cd "$$dir"; \
-		mkdir -p bin lib stamp; \
+		mkdir -p bin lib stamp usr/include usr/lib; \
 	); done
 endef