[SP] Corrected build error by changing second declaration of S_Music_f() to S_StopMusic_f()

Another declaration was added for S_Music_f() with commit https://github.com/JACoders/OpenJK/commit/0e66cd33fefbd646cffeaec0e5a3c628a05c9ff1 . The declaration was changed to S_StopMusic_f() as intended.