Use safe wxStrlcpy() instead of strncpy()

This probably doesn't change much in practice, as it's unlikely that
SDL_GetCurrentAudioDriver() returns a 255 character long stting, but is
still safer and avoid gcc 8 warnings about passing exactly the buffer
size to strncpy().

src/unix/sound_sdl.cpp

@@ -215,7 +215,7 @@ bool wxSoundBackendSDL::OpenAudio()
 #if SDL_MAJOR_VERSION == 1
             SDL_AudioDriverName(driver, 256);
 #elif SDL_MAJOR_VERSION > 1            
-            strncpy(driver, SDL_GetCurrentAudioDriver(), 256);
+            wxStrlcpy(driver, SDL_GetCurrentAudioDriver(), 256);
 #endif
             wxLogTrace(wxT("sound"), wxT("opened audio, driver '%s'"),
                        wxString(driver, wxConvLocal).c_str());