glx/apple: include util/debug.h for env_var_as_boolean prototype

mesa/src/glx/glxcmds.c:1295:21: error: implicit declaration of function 'env_var_as_boolean' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
mesa/src/glx/apple/apple_visual.c:85:28: error: implicit declaration of function 'env_var_as_boolean' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit b37b7b42)