fontsrv: fix on X11 when X11H is not defined

R=rsc
CC=plan9port.codebot
https://codereview.appspot.com/7228044
diff --git a/src/cmd/fontsrv/freetyperules.sh b/src/cmd/fontsrv/freetyperules.sh
index 7b9a002..5ca9157 100644
--- a/src/cmd/fontsrv/freetyperules.sh
+++ b/src/cmd/fontsrv/freetyperules.sh
@@ -2,7 +2,7 @@
 
 if [ "x$1" = "xx11" ]; then
 	if [ "x$2" = "x" ]; then
-		i=/usr/include
+		i="-I/usr/include"
 	else
 		i=$2
 	fi