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: 2ea15cd6 ("d3d12: introduce d3d12 gallium driver")