Use MOZILLA_DIR instead of topsrcdir so that --enable-xterm-updates works in the comm builds.

Bugscape #12347 r=bryner sr=darin

config/rules.mk

@@ -332,7 +332,7 @@ ifdef MOZ_UPDATE_XTERM
 # Its good not to have a newline at the end of the titlebar string because it
 # makes the make -s output easier to read.  Echo -n does not work on all
 # platforms, but we can trick sed into doing it.
-UPDATE_TITLE = sed -e "s!Y!$@ in $(shell $(BUILD_TOOLS)/print-depth-path.sh)/$$d!" $(topsrcdir)/config/xterm.str;
+UPDATE_TITLE = sed -e "s!Y!$@ in $(shell $(BUILD_TOOLS)/print-depth-path.sh)/$$d!" $(MOZILLA_DIR)/config/xterm.str;
 endif
 
 EXIT_ON_ERROR := set -e; # Shell loops continue past errors without this.