Update 'flake.nix'

adding librvg as a flake dependency

flake.nix

@@ -20,7 +20,7 @@
           src = self;
 
           nativeBuildInputs = [ pkgs.meson pkgs.ninja pkgs.pkg-config ];
-          buildInputs = [ pkgs.gtkmm4 pkgs.libsigcxx30 pkgs.glibmm ];
+          buildInputs = [ pkgs.gtkmm4 pkgs.libsigcxx30 pkgs.glibmm pkgs.librsvg ];
 
           meta = with pkgs.lib; {
             description = "A simple GTKmm4 Minesweeper game";
@@ -33,7 +33,7 @@
         # Dev shell for hacking
         devShells.default = pkgs.mkShell {
           nativeBuildInputs = [ pkgs.meson pkgs.ninja pkgs.pkg-config ];
-          buildInputs = [ pkgs.gtkmm4 pkgs.libsigcxx30 pkgs.glibmm ];
+          buildInputs = [ pkgs.gtkmm4 pkgs.libsigcxx30 pkgs.glibmm pkgs.librsvg ];
         };
 
       });