Earlier commit reworked the header install rules, to ensure that the
correct ones are installed only as needed.

By doing so it dropped a wildcard which was effectively including the
wglext.h header in the tarball.

Add the header to the top-level noinst_HEADERS, since the it is not
meant to be installed (autoconf is not used on Windows plaforms).

Fixes: a89faa20

 ("autoconf: Make header install distinct for various
APIs (v2)")
Cc: "12.0 13.0" <mesa-stable@lists.freedesktop.org>
Cc: Chuck Atkins <chuck.atkins@kitware.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>