Fix a folly build failure under clang: ConvTest.cpp.

Summary: [Folly] Fix a folly build failure under clang: ConvTest.cpp.

Test Plan: Build the unit-test folly/test/ConvTest.cpp with clang.

Reviewed By: tudorb@fb.com, njormrod@fb.com

Subscribers: mathieubaudet, dougw

FB internal diff: D1446232

Tasks: 4723132

folly/test/ConvTest.cpp

@@ -936,6 +936,7 @@ void u2aAppendFollyBM(unsigned int n, uint64_t value) {
 
 template <class String>
 struct StringIdenticalToBM {
+  StringIdenticalToBM() {}
   void operator()(unsigned int n, size_t len) const {
     String s;
     BENCHMARK_SUSPEND { s.append(len, '0'); }
@@ -948,6 +949,7 @@ struct StringIdenticalToBM {
 
 template <class String>
 struct StringVariadicToBM {
+  StringVariadicToBM() {}
   void operator()(unsigned int n, size_t len) const {
     String s;
     BENCHMARK_SUSPEND { s.append(len, '0'); }