nir/nir_variable: Add a descriptor set field

We need this for SPIR-V

src/glsl/nir/nir.h

@@ -296,6 +296,11 @@ typedef struct {
        */
       int index;
 
+      /**
+       * Descriptor set binding for sampler or UBO.
+       */
+      int descriptor_set;
+
       /**
        * Initial binding point for a sampler or UBO.
        *