fdm: add missing quote mark in Makefile

This commit will fix compile warning as below

tmp/.config-package.in:22096:warning: multi-line strings not supported

Signed-off-by: Shuoyao Wang <wong.syrone@gmail.com>

mail/fdm/Makefile

@@ -48,7 +48,7 @@ define Build/Prepare
 endef
 
 define Package/fdm/config
-	source "$(SOURCE)/Config.in
+	source "$(SOURCE)/Config.in"
 endef
 
 define Package/fdm/conffiles