meson: fix building without GL

meson: fix building without GL

libgl will be undefined _glx, so move that check inside the
`if with_glx != 'disabled'` block.

v2: - Simplify commit message (Eric, Emil)

Fixes: 5c460337fd9c109 ("meson: Fix GL and EGL pkg-config files with glvnd")
Reported-by: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
CC: Daniel Stone <daniels@collabora.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Untested-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>