gtkdialog: pull gentoo patch for -fno-common toolchains

Without the change build fails on -fno-common toolchains like upstream
gcc-10:

    $ nix build -L --impure --expr 'with import ./. {}; gtkdialog.overrideAttrs (oa: {   NIX_CFLAGS_COMPILE = (oa.NIX_CFLAGS_COMPILE or []) ++ ["-fno-common"]; })'
    ...
    ld: widget_window.o:/build/gtkdialog-0.8.3/src/gtkdialog.h:43:
      multiple definition of `function_signals_block'; gtkdialog.o:/build/gtkdialog-0.8.3/src/gtkdialog.h:43: first defined here

pkgs/development/tools/misc/gtkdialog/default.nix

@@ -1,4 +1,4 @@
-{lib, stdenv, fetchurl, gtk2, pkg-config }:
+{lib, stdenv, fetchurl, fetchpatch, gtk2, pkg-config }:
 
 stdenv.mkDerivation rec {
   pname = "gtkdialog";
@@ -8,6 +8,14 @@ stdenv.mkDerivation rec {
     url = "https://storage.googleapis.com/google-code-archive-downloads/v2/code.google.com/${pname}/${pname}-${version}.tar.gz";
     sha256 = "ff89d2d7f1e6488e5df5f895716ac1d4198c2467a2a5dc1f51ab408a2faec38e";
   };
+  patches = [
+    # Pull Gentoo patch for -fno-common toolchain fix.
+    (fetchpatch {
+      name = "fno-common.patch";
+      url = "https://gitweb.gentoo.org/repo/gentoo.git/plain/x11-misc/gtkdialog/files/gtkdialog-0.8.3-fno-common.patch?id=98692e4c4ad494b88c4902ca1ab3e6541190bbe8";
+      sha256 = "1mh01krzpfy7lbbqx3xm71xsiqqgg67w4snv794wspfqkk2hicvz";
+    })
+  ];
 
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [ gtk2 ];