dl_win32.xs: can't assign literal to "char *"

This generated code like:

        char *	filename;

	if (items < 3)
	    filename = "DynaLoader";
	else {
	    filename = (char *)SvPV_nolen(ST(2))

which failed in C++ builds (and isn't valid in C either), when
assigning a "const char *" (well, "const char [11]") to "char *".

ext/DynaLoader/dl_win32.xs

@@ -188,7 +188,7 @@ void
 dl_install_xsub(perl_name, symref, filename="$Package")
     char *		perl_name
     void *		symref 
-    char *		filename
+    const char *	filename
     CODE:
     DLDEBUG(2,PerlIO_printf(Perl_debug_log,"dl_install_xsub(name=%s, symref=%x)\n",
 		      perl_name, symref));