linux-rpi: Fix modDirVersion

This causes build breakage on staging due to #7524.

pkgs/os-specific/linux/kernel/linux-rpi.nix

···
 in import ./generic.nix (args // rec {
   version = "3.18.y-${rev}";
 
-  modDirVersion = "3.18.7-v7";
+  modDirVersion = "3.18.7";
 
   src = fetchurl {
     url = "https://api.github.com/repos/raspberrypi/linux/tarball/${rev}";