fix Linux build

Otherwise a build will fail with this message:
```
In file included from /build/buildd/aegisub-unstable-8591/src/preferences.cpp:37:0:
/build/buildd/aegisub-unstable-8591/src/audio_player_portaudio.h:98:32: error: expected ')' before '*' token
  PortAudioPlayer(AudioProvider *provider);
                                ^
cc1plus: warning: unrecognized command line option "-Wno-c++11-narrowing" [enabled by default]
make[1]: *** [/build/buildd/aegisub-unstable-8591/src/preferences.o] Error 1
make[1]: Leaving directory `/build/buildd/aegisub-unstable-8591'
dh_auto_build: make -j1 returned exit code 2
make: *** [build-arch] Error 2
dpkg-buildpackage: error: debian/rules build-arch gave error exit status 2
```