vkd3d-shader: Allow static const variables to lack an initializer.

Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>
Signed-off-by: Matteo Bruni <mbruni@codeweavers.com>
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

tests/hlsl-invalid.shader_test

@@ -115,3 +115,10 @@ float4 main() : sv_target
 {
     return 0;
 }
+
+[pixel shader fail]
+float4 main() : sv_target
+{
+    const float4 x;
+    return x;
+}