Update ArchLinux PKGBUILD to clone submodules

dist/archlinux/PKGBUILD

@@ -2,7 +2,7 @@
 # Maintainer: Markus Koch <markus@notsyncing.net>
 
 pkgname=mdiary-git
-pkgver=20180317.126.a5e5cd7
+pkgver=20180708.137.017e6e8
 pkgrel=1
 pkgdesc='A viewer for a file based diary with entries written in markdown.'
 arch=('i686' 'x86_64')
@@ -19,6 +19,11 @@ pkgver () {
 	echo "$_date.$(git rev-list --count master).$(git rev-parse --short master)"
 }
 
+prepare() {
+	cd $pkgname
+	git submodule init
+	git submodule update
+}
 
 build() {
 	cd "$srcdir/$pkgname"