games/spring: Unbreak with openal-soft-1.20.0

In file included from rts/System/Sound/OpenAL/Sound.cpp:7:
/usr/local/include/AL/alc.h:34:16: error: typedef 'ALCdevice' cannot be referenced with a struct specifier
typedef struct ALCdevice ALCdevice;

^

rts/System/Sound/OpenAL/Sound.h:23:33: note: declared here
typedef struct ALCdevice_struct ALCdevice;

^

1 error generated.

http://beefy6.nyi.freebsd.org/data/120amd64-default/518685/logs/errors/spring-98.0_19.log