qsynth: disable install parallelism

Without the change parallel installs fail as:

lrelease error: Parse error at src/translations/qsynth_ru.ts:1503:33: Premature end of document.
make: *** [Makefile:107: src/translations/qsynth_ru.qm] Error 1

+4
+4
pkgs/applications/audio/qsynth/default.nix
··· 16 16 buildInputs = [ alsa-lib fluidsynth libjack2 qtbase qttools qtx11extras ]; 17 17 18 18 enableParallelBuilding = true; 19 + # Missing install depends: 20 + # lrelease error: Parse error at src/translations/qsynth_ru.ts:1503:33: Premature end of document. 21 + # make: *** [Makefile:107: src/translations/qsynth_ru.qm] Error 1 22 + enableParallelInstalling = false; 19 23 20 24 meta = with lib; { 21 25 description = "Fluidsynth GUI";