meson: detect winflex/bison only on native win32

we want to detect the native bison when cross-compiling

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9375>

meson.build

@@ -1708,7 +1708,7 @@ else
   asan_c_args = ['-DBUILT_WITH_ASAN=0']
 endif
 
-if host_machine.system() == 'windows'
+if build_machine.system() == 'windows'
   # Prefer the winflexbison versions, they're much easier to install and have
   # better windows support.