gallium/swr: Fix building swr with MSVC

Fix building swr with MSVC by turning off
UNICODE before including windows.h.

Reviewed-by: Jan Zielinski <jan.zielinski@intel.com>
Part-of: <mesa/mesa!5166>
24 jobs for !5166 with swr_fix_msvc19_build in 12 minutes and 54 seconds (queued for 2 seconds)
merge request