meson: don't build gallium trivial tests on windows

They require the pipe-loaders, which require xmlconfig, which doesn't
build with msvc.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Kristian H. Kristensen <hoegsberg@google.com>

src/gallium/tests/meson.build

@@ -18,6 +18,11 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
-subdir('trivial')
-subdir('unit')
+if not with_platform_windows
+  # pipe-loader doesn't build on windows.
+  subdir('trivial')
+endif
+if with_gallium_softpipe
+  subdir('unit')
+endif
 subdir('graw')