sdl_sound: add license info

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/sdl_sound/sdl_sound.mk

@@ -7,6 +7,8 @@
 SDL_SOUND_VERSION = 1.0.3
 SDL_SOUND_SOURCE = SDL_sound-$(SDL_SOUND_VERSION).tar.gz
 SDL_SOUND_SITE = http://icculus.org/SDL_sound/downloads/
+SDL_SOUND_LICENSE = LGPLv2.1+
+SDL_SOUND_LICENSE_FILES = COPYING
 SDL_SOUND_INSTALL_STAGING = YES
 SDL_SOUND_DEPENDENCIES = sdl