[rg3igalia]

The current code uses #if _WIN32, which works but causes warnings when compiling with certain compilers and flags (see -Wundef for GCC and Clang). This commit replaces the checks with #ifdef _WIN32, which does not produce any warning.

[rg3igalia]

Note: this fix comes from VK-GL-CTS now pulling shaderc as a dependency, resulting in the following warnings:

[548/2015] Building CXX object _deps/shaderc-build/libshaderc_util/CMakeFiles/shaderc_util.dir/src/io_shaderc.cc.o
/home/rgarcia/devel/vk-gl-cts/build/_deps/shaderc-src/libshaderc_util/src/io_shaderc.cc:19:5: warning: '_WIN32' is not defined, evaluates to 0 [-Wundef]
   19 | #if _WIN32
      |     ^
/home/rgarcia/devel/vk-gl-cts/build/_deps/shaderc-src/libshaderc_util/src/io_shaderc.cc:134:5: warning: '_WIN32' is not defined, evaluates to 0 [-Wundef]
  134 | #if _WIN32
      |     ^
/home/rgarcia/devel/vk-gl-cts/build/_deps/shaderc-src/libshaderc_util/src/io_shaderc.cc:141:5: warning: '_WIN32' is not defined, evaluates to 0 [-Wundef]
  141 | #if _WIN32
      |     ^
3 warnings generated.