build: add missing dependency

Commit f745b55 missed install-tools' dependency on
build-tools-public-headers.

Discovered by Travis-CI.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>

Makefile

@@ -103,7 +103,7 @@ install-xen:
 	$(MAKE) -C xen install
 
 .PHONY: install-tools
-install-tools:
+install-tools: build-tools-public-headers
 	$(MAKE) -C tools install
 
 .PHONY: install-stubdom