spirv: Explicitly break when finished handling SpvDecorationBuiltIn

When tyding up this section in 1e5b09f42f ("spirv: Tidy some repeated
if checks by using a switch statement.") the break got lost.  It is
not a real problem because the next case just break, but better to
have it explicitly here instead of a FALLTHROUGH.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9440>

src/compiler/spirv/vtn_variables.c

@@ -1200,7 +1200,8 @@ apply_var_decoration(struct vtn_builder *b,
       default:
          break;
       }
-      FALLTHROUGH;
+
+      break;
    }
 
    case SpvDecorationSpecId: