frontend/va: Fix WIN32 VA_DRIVER_INIT_FUNC declaration. Remove declspec as it uses .def file
Fixes:
b557ceb7 ("frontends/va: Add windows VA frontend support via vl_winsys_win32 and libva-win32")
Closes: #7702
Signed-off-by: Sil Vilerino <sivileri@microsoft.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19754>
(cherry picked from commit
d81e40b20c386a79d643fc6c14c3cfb8351daa09)