fix phpPackages memcache,memcached,xdebug

These packages, and maybe some more include unix.h for some reason.
Creating that file makes them build, and in the case of xdebug also
appear to work.

pkgs/build-support/build-pecl.nix

@@ -20,4 +20,6 @@ stdenv.mkDerivation (args // {
   makeFlags = [ "EXTENSION_DIR=$(out)/lib/php/extensions" ] ++ makeFlags;
 
   autoreconfPhase = "phpize";
+
+  preConfigure = "touch unix.h";
 })