fixed XPI generation, thanks teramako

Makefile.common

@@ -16,6 +16,7 @@ JAR_TXT_FILES = ${shell find -L content skin locale	\
 			     -o -path '*.css'	\
 			     -o -path '*.xul'	\
 			     -o -path '*.html'	\
+ 			     -o -path '*.xhtml'	\
 			   \)			\
 		}
 JAR_DIRS      = $(foreach f,${JAR_FILES},$(dir $f))