GH-425 Avoid C++ compiler warning about conversion from string litera…

…l to 'char *.

In C string literals do not have the 'const' qualifier but in C++ they do.
Avoid C++ compiler warnings by changing our TRACE() funciton definition to use
'const char *' instead of plain 'char *'. This is a useful change for C too
because they string passed to the function can be expected to be read-only.

SSLeay.xs

@@ -227,7 +227,7 @@ which conflicts with perls
 #define PR4(s,t,u,v)
 #endif
 
-static void TRACE(int level,char *msg,...) {
+static void TRACE(int level,const char *msg,...) {
     va_list args;
     SV *trace = get_sv("Net::SSLeay::trace",0);
     if (trace && SvIOK(trace) && SvIV(trace)>=level) {