ppsspp: fix qmake4Hook build pass PPSSPPQt.pro which was removed in 34625f6

pkgs/misc/emulators/ppsspp/default.nix

···
   buildInputs = [ zlib libpng pkgconfig qt4 qmake4Hook ]
                 ++ (if withGamepads then [ SDL ] else [ ]);
 
+  qmakeFlags = [ "PPSSPPQt.pro" ];
+
   preConfigure = "cd Qt";
   installPhase = "mkdir -p $out/bin && cp ppsspp $out/bin";