pjproject_bundled: Fix issue where "/version.mak" wasn't found

main/Makefile includes third-party/pjproject/build.mak but
doesn't set PJDIR beforehand so "include $(PJDIR)/version.mak"
evaluates to "/version.mak".  Fix is to set PJDIR in main/Makefile
before the include.

Change-Id: I0f7c67d60209049056fe9c4b041bf0463aa95604

main/Makefile

@@ -253,6 +253,7 @@ ifeq ($(PJPROJECT_BUNDLED),yes)
 ASTPJ_SO_VERSION=2
 ASTPJ_LDLIBS=-L. -lasteriskpj
 
+PJDIR=$(ASTTOPDIR)/$(PJPROJECT_DIR)/source
 -include $(ASTTOPDIR)/$(PJPROJECT_DIR)/build.mak
 
 PJPROJECT_LDLIBS := \