st/mesa: fix incorrect pointer type arguments in st_new_program()

st/mesa: fix incorrect pointer type arguments in st_new_program()

Silences 5 warnings of the type:
state_tracker/st_cb_program.c: In function 'st_new_program':
state_tracker/st_cb_program.c:108:7: warning: passing argument 1 of
'_mesa_init_gl_program' from incompatible pointer type [enabled by default]
       return _mesa_init_gl_program(&prog->Base, target, id);
       ^
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>