postmarketos-initramfs: depend on mdevd

I somehow hit a case where this wasn't installed and
/lib/mdev/persistent-storage wasn't available. Let's just explicitly
depend on mdevd...

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

main/postmarketos-initramfs/APKBUILD

@@ -21,6 +21,7 @@ depends="
 	f2fs-tools
 	iskey
 	lz4
+	mdevd
 	mdev-conf
 	multipath-tools
 	parted