d3d12: fix warnings for missing-braces on clang

d3d12: fix warnings for missing-braces on clang

[3485/3754] Compiling C++ object src/gallium/drivers/d3d12/libd3d12.a.p/d3d12_screen.cpp.o
../src/gallium/drivers/d3d12/d3d12_screen.cpp:56:65: warning: suggest braces around initialization of subobject [-Wmissing-braces]
static GUID OpenGLOn12CreatorID = { 0x6bb3cd34, 0x0d19, 0x45ab, 0x97, 0xed, 0xd7, 0x20, 0xba, 0x3d, 0xfc, 0x80 };
                                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                                {                                             }

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18307>