hhvm: fix build with gcc7

+1
+1
pkgs/development/compilers/hhvm/default.nix
··· 53 53 hphp/runtime/ext/std/*.cpp \ 54 54 hphp/runtime/ext_zend_compat/php-src/main/*.cpp \ 55 55 hphp/runtime/ext_zend_compat/php-src/main/*.h 56 + sed '1i#include <functional>' -i third-party/mcrouter/src/mcrouter/lib/cycles/Cycles.h 56 57 patchShebangs . 57 58 ''; 58 59