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)
detached
Status Name Job ID Coverage
  Container
passed arm_build #2792514
aarch64

00:00:52

passed arm_test #2792516
aarch64

00:00:21

passed windows_build_vs2019 #2792517
windows shell 1809 mesa

00:01:56

passed x86_build #2792510

00:01:25

passed x86_build_old #2792513

00:00:20

passed x86_cross_arm_test #2792515

00:00:41

passed x86_test-gl #2792511

00:00:41

passed x86_test-vk #2792512

00:00:48

 
  Meson X86 64
passed meson-clang #2792521

00:07:16

passed meson-classic #2792520

00:05:06

passed meson-clover #2792522

00:09:05

passed meson-clover-old-llvm #2792523

00:05:10

passed meson-gallium #2792519

00:11:27

passed meson-testing #2792518

00:07:50

passed meson-vulkan #2792524

00:06:42

 
  Scons
passed scons-win64 #2792525

00:05:44

 
  Meson Misc
passed meson-arm64 #2792527
aarch64

00:07:12

passed meson-arm64-build-test #2792528
aarch64

00:07:14

passed meson-armhf #2792526
aarch64

00:07:51

passed meson-i386 #2792530

00:05:02

passed meson-mingw32-x86_64 #2792533

00:04:51

passed meson-ppc64el #2792532
kvm

00:06:15

passed meson-s390x #2792531
kvm

00:06:17

passed meson-windows-vs2019 #2792529
windows docker 1809 mesa

00:07:55