gqview: add -fcommon workaround

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

    $ nix build -L --impure --expr 'with import ./. {}; gqview.overrideAttrs (oa: {   NIX_CFLAGS_COMPILE = (["-fno-common"] ++ [oa.NIX_CFLAGS_COMPILE or ""]); })'
    ...
    ld: pan-view.o:/build/gqview-2.1.5/src/bar_exif.h:27:
      multiple definition of `bar_exif_key_list'; bar_exif.o:/build/gqview-2.1.5/src/bar_exif.h:27: first defined here

pkgs/applications/graphics/gqview/default.nix

@@ -15,6 +15,7 @@ stdenv.mkDerivation rec {
 
   hardeningDisable = [ "format" ];
 
+  NIX_CFLAGS_COMPILE = "-fcommon";
   NIX_LDFLAGS = "-lm";
 
   meta = with lib; {