frr: fix cross compilation

protobufc is also needed as nativeBuildInput in order for
cross compilation to work. Tested for aarch64.

Signed-off-by: Markus Theil <[e-mail]>

pkgs/servers/frr/default.nix

···
     pkg-config
     python3.pkgs.sphinx
     texinfo
+    protobufc
   ];
 
   buildInputs = [