Fix leaking modifiers string.
Reported by valgrind:
==118175== 17 bytes in 1 blocks are definitely lost in loss record 13 of 1,675
==118175== at 0x483A809: malloc (vg_replace_malloc.c:307)
==118175== by 0x5CD1B46: _XlcDefaultMapModifiers (in /usr/lib64/libX11.so.6.3.0)
==118175== by 0x5CD1F1A: XSetLocaleModifiers (in /usr/lib64/libX11.so.6.3.0)
==118175== by 0x496841C: X11_InitKeyboard (SDL_x11keyboard.c:324)
==118175== by 0x496F0CA: X11_VideoInit (SDL_x11video.c:455)
==118175== by 0x494747B: SDL_VideoInit_REAL (SDL_video.c:532)
==118175== by 0x489E886: SDL_InitSubSystem_REAL (SDL.c:206)
==118175== by 0x402634: main (fade.cc:35)