Add #include <cerrno> in verilated.h

Related to the error of verilator#2255:
/usr/bin/ld: _ZN9Verilated3t_sE: TLS reference in ... mismatches non-TLS definition in verilated.o section .bss

Adding `#include <errno.h>` does not seem to provide a consistent fix - I think that the `#include <cerrno>` of verilated.cpp provides a conflicting definition of `errno` in some edge-cases. Adding the identical include in verilated.h (seems to) fix the error.

include/verilated.h

@@ -45,6 +45,7 @@
 #include <memory>
 #include <string>
 #include <vector>
+#include <cerrno>
 // <iostream> avoided to reduce compile time
 // <map> avoided and instead in verilated_heavy.h to reduce compile time
 // <string> avoided and instead in verilated_heavy.h to reduce compile time