hhvm: fix build

Upstream re2 commit:

https://github.com/google/re2/commit/b94b7cd42e9f02673cd748c1ac1d16db4052514c

Upstream bytecode.h commit:

https://github.com/facebook/hhvm/commit/f8a75ca667a1d27be2a304502edd01d064cce20a

pkgs/development/compilers/hhvm/default.nix

@@ -26,6 +26,10 @@ stdenv.mkDerivation rec {
       gmp libyaml libedit libvpx imagemagick fribidi gperf
     ];
 
+  patches = [
+    ./flexible-array-members-gcc6.patch
+  ];
+
   enableParallelBuilding = false; # occasional build problems;
   dontUseCmakeBuildDir = true;
   NIX_LDFLAGS = "-lpam -L${pam}/lib";