meson: verify that d3d12.h exists when building the d3d12 driver

Without this header-file, we can't build the driver. So let's verify
that it exists, and can be used by the C++ compiler.

This should make it a bit more clear what's wrong if someone attempts to
build this using MinGW or on Linux.

Fixes: 2ea15cd661c ("d3d12: introduce d3d12 gallium driver")
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7575>
1 file changed