Fix to include limits header for std::numeric_limits (#2747)

Code using `std::numeric_limits` wasn't including the limits header,
leading to a compilation error in Fedora Rawhide (GCC 11.0).

include/verilated.cpp

@@ -32,6 +32,7 @@
 #include <sstream>
 #include <sys/stat.h>  // mkdir
 #include <list>
+#include <limits>
 #include <utility>
 
 // clang-format off

src/V3Trace.cpp

@@ -46,6 +46,7 @@
 #include "V3Stats.h"
 
 #include <map>
+#include <limits>
 #include <set>
 
 //######################################################################