Add ACL as a common initialism.

Signed-off-by: Joe Tsai <joetsai@google.com>
diff --git a/lint.go b/lint.go
index ab6433a..8dc4ae5 100644
--- a/lint.go
+++ b/lint.go
@@ -763,6 +763,7 @@
 	"UTF8":  true,
 	"VM":    true,
 	"XML":   true,
+	"XMPP":  true,
 	"XSRF":  true,
 	"XSS":   true,
 }