Add zlib as a link directive on posix systems for the install script.

This technically should be provided by phobos, but on some default os
packages, zlib is not included with phobos, and you must explicitly link
against it.

install/dub.json

@@ -9,6 +9,7 @@
         "io": "~>0.3.4",
         "getopt2": "0.0.1"
     },
+    "libs-posix" : ["z"],
     "description": "Install raylib binary files",
     "license": "boost-1.0",
     "name": "install"