make tvm compilable by gcc 4.9.2 (#4032)

please see https://stackoverflow.com/a/26949099

src/op/tensorize.cc

@@ -522,7 +522,7 @@ TVM_REGISTER_API("test.op.MatchTensorizeBody")
     CHECK(stage->op.as<ComputeOpNode>());
     *ret = MatchTensorizeBody(stage->op.as<ComputeOpNode>(),
                               stage,
-                              {},
+                              {{}},
                               as_unordered_map(out_dom),
                               as_unordered_map(in_region),
                               intrin,

src/relay/pass/well_formed.cc

@@ -43,7 +43,7 @@ class WellFormedChecker : private ExprVisitor, PatternVisitor {
   struct Scope {
     WellFormedChecker* wfc;
     explicit Scope(WellFormedChecker* wfc) : wfc(wfc) {
-      wfc->scope.push_back({});
+      wfc->scope.push_back({{}});
     }
     ~Scope() {
       CHECK_GE(wfc->scope.size(), 0);