spirv: Shut up unhandled enumeration value warnings.

We don't want to do anything for the other cases.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
diff --git a/src/compiler/spirv/vtn_variables.c b/src/compiler/spirv/vtn_variables.c
index 3ecb54f..1cc1402 100644
--- a/src/compiler/spirv/vtn_variables.c
+++ b/src/compiler/spirv/vtn_variables.c
@@ -1078,6 +1078,8 @@
       case SpvBuiltInFragCoord:
          nir_var->data.pixel_center_integer = b->pixel_center_integer;
          break;
+      default:
+         break;
       }
    }