Suppress -Wformat-overflow warning/error

GCC 7 and later warn about overflow/truncation when using
sprintf and related functions with fixed-size buffers.

Suppress the warning in schematest.cpp.

test/perftest/schematest.cpp

@@ -11,6 +11,11 @@
 
 using namespace rapidjson;
 
+RAPIDJSON_DIAG_PUSH
+#if defined(__GNUC__) && __GNUC__ >= 7
+RAPIDJSON_DIAG_OFF(format-overflow)
+#endif
+
 template <typename Allocator>
 static char* ReadFile(const char* filename, Allocator& allocator) {
     const char *paths[] = {
@@ -42,6 +47,8 @@ static char* ReadFile(const char* filename, Allocator& allocator) {
     return json;
 }
 
+RAPIDJSON_DIAG_POP
+
 class Schema : public PerfTest {
 public:
     Schema() {}