subsurface: disable parallel make

It was failing on Hydra:
http://hydra.nixos.org/build/48847277/nixlog/2/raw

pkgs/applications/misc/subsurface/default.nix

···
   ];
   nativeBuildInputs = [ cmake pkgconfig ];
 
-  enableParallelBuilding = true;
+  enableParallelBuilding = false; # subsurfacewebservices.h dependency on ui_webservices.h
 
   cmakeFlags = [
     "-DMARBLE_LIBRARIES=${libmarble}/lib/libssrfmarblewidget.so"