gallivm: Re-enable LLVMUnionTypeKind case for llvm-2.7 only.

LLVMUnionTypeKind is not in llvm-2.6, llvm-2.8, llvm-2.9, or llvm-3.0svn.

src/gallium/auxiliary/gallivm/lp_bld_type.c

@@ -333,10 +333,10 @@ lp_typekind_name(LLVMTypeKind t)
       return "LLVMVectorTypeKind";
    case LLVMMetadataTypeKind:
       return "LLVMMetadataTypeKind";
-   /* Only in LLVM 2.7 and later???
+#if HAVE_LLVM == 0x0207
    case LLVMUnionTypeKind:
       return "LLVMUnionTypeKind";
-   */
+#endif
    default:
       return "unknown LLVMTypeKind";
    }