tools: correct build in directory below tools

Recent changes to create *.pc files introduced a bug when trying to
build a library from a directory below tools as PKG_CONFIG_DIR wouldn't
be set. Correct this by adding a default value to Rules.mk.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/Rules.mk

@@ -249,6 +249,8 @@ $(XEN_ROOT)/config/Tools.mk:
 	$(error You have to run ./configure before building or installing the tools)
 endif
 
+PKG_CONFIG_DIR ?= $(XEN_ROOT)/tools/pkg-config
+
 $(PKG_CONFIG_DIR)/%.pc: %.pc.in Makefile
 	mkdir -p $(PKG_CONFIG_DIR)
 	@sed -e 's!@@version@@!$(PKG_CONFIG_VERSION)!g' \