extraModprobeConfig: set type to types.lines

This makes it so multiple definitions are merged by adding a newline
between each entry, to avoid the need to add a newline to the end of
every definition of extraModprobeConfig. See #119 for an example of an
issue this has caused.

Signed-off-by: Shea Levy <shea@shealevy.com>

modules/system/boot/modprobe.nix

@@ -57,6 +57,7 @@ with pkgs.lib;
         <citerefentry><refentrytitle>modprobe.conf</refentrytitle>
         <manvolnum>5</manvolnum></citerefentry> for details.
       '';
+      type = types.lines;
     };
 
   };