build: Use https in submodule URL for external/xdg-file-dialog

Author: hornang

.gitmodules

@@ -15,7 +15,7 @@
 	url = https://github.com/hornang/MercatorTile
 [submodule "external/xdg-file-dialog"]
 	path = external/xdg-file-dialog
-	url = http://github.com/hornang/xdg-file-dialog.git
+	url = https://github.com/hornang/xdg-file-dialog.git
 [submodule "external/msdf-atlas-gen"]
 	path = external/msdf-atlas-gen
 	url = https://github.com/Chlumsky/msdf-atlas-gen.git