build: don't require full tools build for building stubdoms

With the drop of qemu-traditional "make stubdom" no longer requires
"make tools" to have finished.

It is enough to add "install-tools-public-headers" as a prereq of
"install-stubdom".

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>

Makefile

@@ -147,7 +147,7 @@ install-tools: install-tools-public-headers
 	$(MAKE) -C tools install
 
 .PHONY: install-stubdom
-install-stubdom: mini-os-dir install-tools
+install-stubdom: mini-os-dir install-tools-public-headers
 	$(MAKE) -C stubdom install
 ifeq (x86_64,$(XEN_TARGET_ARCH))
 	XEN_TARGET_ARCH=x86_32 $(MAKE) -C stubdom install-grub-if-enabled