xpwn: add -fcommon workaround

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

    ld: ../ipsw-patch/libxpwn.a(libxpwn.c.o):(.bss+0x4): multiple definition of
      `endianness'; CMakeFiles/xpwn-bin.dir/src/xpwn.cpp.o:(.bss+0x0): first defined here

pkgs/development/mobile/xpwn/default.nix

@@ -11,6 +11,11 @@ stdenv.mkDerivation rec {
     sha256 = "1qw9vbk463fpnvvvfgzxmn9add2p30k832s09mlycr7z1hrh3wyf";
   };
 
+  # Workaround build failure on -fno-common toolchains:
+  #   ld: ../ipsw-patch/libxpwn.a(libxpwn.c.o):(.bss+0x4): multiple definition of
+  #     `endianness'; CMakeFiles/xpwn-bin.dir/src/xpwn.cpp.o:(.bss+0x0): first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   preConfigure = ''
     rm BUILD # otherwise `mkdir build` fails on case insensitive file systems
     sed -r -i \