git2r: add SSH support

When building the devtools R module, which depends on git2r, it reports
that libssh2 cannot be found, and falls back to building without SSH
support. Adding libssh2.dev to git2r's native build inputs fixes this.

pkgs/development/r-modules/default.nix

@@ -258,7 +258,7 @@ let
     Formula = [ pkgs.gmp ];
     geoCount = [ pkgs.gsl_1 ];
     gdtools = [ pkgs.cairo.dev pkgs.fontconfig.lib pkgs.freetype.dev ];
-    git2r = [ pkgs.zlib.dev pkgs.openssl.dev ];
+    git2r = [ pkgs.zlib.dev pkgs.openssl.dev pkgs.libssh2.dev ];
     GLAD = [ pkgs.gsl_1 ];
     glpkAPI = [ pkgs.gmp pkgs.glpk ];
     gmp = [ pkgs.gmp.dev ];