directvnc: pull -fno-common fix pending upstream inclusion

Without the change build fails on upstream gcc-10 as:

    ld: modmap.o:/build/source/src/directvnc.h:45: multiple definition of
      `buffer'; main.o:/build/source/src/directvnc.h:45: first defined here

pkgs/os-specific/linux/directvnc/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, autoreconfHook, pkg-config, directfb, zlib, libjpeg, xorgproto }:
+{ lib, stdenv, fetchFromGitHub, fetchpatch, autoreconfHook, pkg-config, directfb, zlib, libjpeg, xorgproto }:
 
 stdenv.mkDerivation {
   pname = "directvnc";
@@ -11,6 +11,17 @@ stdenv.mkDerivation {
     sha256 = "16x7mr7x728qw7nbi6rqhrwsy73zsbpiz8pbgfzfl2aqhfdiz88b";
   };
 
+  patches = [
+    # Pull fix pending upstream inclusion for -fno-common toolchain
+    # support:
+    #   https://github.com/drinkmilk/directvnc/pull/7
+    (fetchpatch {
+      name = "fno-common.patch";
+      url = "https://github.com/drinkmilk/directvnc/commit/e9c23d049bcf31d0097348d44391fe5fd9aad12b.patch";
+      sha256 = "1dnzr0dnx20w80r73j4a9n6mhbazjzlr5ps9xjj898924cg140zx";
+    })
+  ];
+
   nativeBuildInputs = [ autoreconfHook pkg-config ];
 
   buildInputs = [ directfb zlib libjpeg xorgproto ];