meson: don't install windows headers on non-windows platforms

Only dive into the windows subdir if windows platform is selected.

Signed-off-by: Marc Dietrich <marvin24@gmx.de>
Fixes: 5ef75cb02b2b4db5506b8 "meson: build src/glx/windows"
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
diff --git a/src/glx/meson.build b/src/glx/meson.build
index 04cd647..e645b22 100644
--- a/src/glx/meson.build
+++ b/src/glx/meson.build
@@ -18,7 +18,9 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
-subdir('windows')
+if with_dri_platform == 'windows'
+  subdir('windows')
+endif
 
 files_libglx = files(
   'clientattrib.c',