r-modules: fix gert build

gert requires pkgs.libgit2 to build.

pkgs/development/r-modules/default.nix

@@ -263,6 +263,7 @@ let
     gmp = [ pkgs.gmp.dev ];
     graphscan = [ pkgs.gsl_1 ];
     gsl = [ pkgs.gsl_1 ];
+    gert = [ pkgs.libgit2 ];
     haven = [ pkgs.libiconv pkgs.zlib.dev ];
     h5vc = [ pkgs.zlib.dev ];
     HiCseg = [ pkgs.gsl_1 ];