Use mutex in ElapsedSeconds() to protect static variable.

Given C++'s rules for initialization of statics, I don't think this is
strictly necessary, but it makes helgrind happy and causes no harm.

src/pbrt/util/log.cpp

@@ -50,6 +50,9 @@ namespace pbrt {
 namespace {
 
 float ElapsedSeconds() {
+    static std::mutex mutex;
+    std::lock_guard<std::mutex> lock(mutex);
+
     using clock = std::chrono::steady_clock;
     static clock::time_point start = clock::now();