Use std::fabs() instead of abs() in rationalTime.cpp (#721)

* Use std::abs() instead of abs()

* Use std::fabs instead of std::abs

src/opentime/rationalTime.cpp

@@ -290,7 +290,7 @@ RationalTime::to_time_string() const {
     // result and return the string at the end with a '-'. This provides
     // compatibility with ffmpeg, which allows negative time strings.
     if (std::signbit(total_seconds)) {
-        total_seconds = abs(total_seconds);
+        total_seconds = std::fabs(total_seconds);
         is_negative = true;
     }