vokoscreen: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:

    ld: alsa_device.o:(.bss+0x8): multiple definition of `rc'; QvkAlsaDevice.o:(.bss+0x8): first defined here

pkgs/applications/video/vokoscreen/default.nix

@@ -30,6 +30,10 @@ mkDerivation rec {
     ./ffmpeg-out-of-box.patch
   ];
 
+  # Workaround build failure on -fno-common toolchains:
+  #   ld: alsa_device.o:(.bss+0x8): multiple definition of `rc'; QvkAlsaDevice.o:(.bss+0x8): first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   preConfigure = ''
     sed -i 's/lrelease-qt5/lrelease/g' vokoscreen.pro
   '';