netbsd.libresolv: fix build

Tested building pkgsCross.x86_64-netbsd.netbsd.libresolv from
x86_64-linux.

pkgs/os-specific/bsd/netbsd/default.nix

···
     version = "8.0";
     sha256 = "11vpb3p2343wyrhw4v9gwz7i0lcpb9ysmfs9gsx56b5gkgipdy4v";
     meta.platforms = lib.platforms.netbsd;
+    extraPaths = [ libc.src ];
   };
 
   libm = mkDerivation {