Added missing import.

sdl.mod/linux_source.bmx

@@ -31,6 +31,7 @@ Import "SDL/src/core/linux/SDL_dbus.c"
 Import "SDL/src/core/linux/SDL_evdev.c"
 Import "SDL/src/core/linux/SDL_ime.c"
 Import "SDL/src/core/linux/SDL_udev.c"
+Import "SDL/src/core/linux/SDL_threadprio.c"
 Import "SDL/src/core/unix/SDL_poll.c"
 Import "SDL/src/filesystem/unix/SDL_sysfilesystem.c"
 Import "SDL/src/haptic/linux/SDL_syshaptic.c"