glx: Fix compilation with GLX_USE_WINDOWSGL

Sadly, the GLX_USE_APPLEGL and GLX_USE_WINDOWSGL cases are not identical
(because GLX_USE_WINDOWSGL uses vtables rather than a maze of ifdefs)

Include <sys/time.h> again, as functions prototyped by it are used in
the GLX_USE_WINDOWSGL path.

Make the include guard around the __glxGetMscRate() definition match the
one at it's declaration again, as it's referenced from dri_common.c
which is built for GLX_USE_WINDOWSGL.

Fixes: a95ec138 ("glx: mandate xf86vidmode only for "drm" dri platforms")
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit d512b35b)