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>
24 jobs for !5166 with swr_fix_msvc19_build
detached
Status Name Job ID Coverage
  Container
manual arm_build #2792490
aarch64 manual
manual arm_test #2792492
aarch64 manual
manual windows_build_vs2019 #2792493
windows shell 1809 mesa manual
manual x86_build #2792486
manual
manual x86_build_old #2792489
manual
manual x86_cross_arm_test #2792491
manual
manual x86_test-gl #2792487
manual
manual x86_test-vk #2792488
manual
 
  Meson X86 64
created meson-clang #2792497
created meson-classic #2792496
created meson-clover #2792498
created meson-clover-old-llvm #2792499
created meson-gallium #2792495
created meson-testing #2792494
created meson-vulkan #2792500
 
  Scons
created scons-win64 #2792501
allowed to fail
 
  Meson Misc
created meson-arm64 #2792503
aarch64
created meson-arm64-build-test #2792504
aarch64
created meson-armhf #2792502
aarch64
created meson-i386 #2792506
created meson-mingw32-x86_64 #2792509
created meson-ppc64el #2792508
kvm
created meson-s390x #2792507
kvm
created meson-windows-vs2019 #2792505
windows docker 1809 mesa