dnsmasq: correct previous change for Darwin

I was a bit hasty in commit 482642a.

pkgs/tools/networking/dnsmasq/default.nix

@@ -27,7 +27,7 @@ stdenv.mkDerivation rec {
       sha256 = "1hnixij3jp1p6zc3bx2dr92yyf9jp1ahhl9hiiq7bkbhbrw6mbic";
     })
   ];
-  postPatch = ''
+  postPatch = stdenv.lib.optionalString stdenv.hostPlatform.isLinux ''
     sed '1i#include <linux/sockios.h>' -i src/dhcp.c
   '';