Fix string-init compile error (g++ 4.4) in llurlentry_stub.cpp.

Newer C++ compilers produce a (fatal) warning when you try to initialize a
(non-const) char* variable with a string literal: you're supposed to capture
such data in a const char*. But as this module is resolving external
references declared in ancient message-system headers, we don't have the
option to fix the original declaration. Instead use
char* _PREHASH_Foo = const_cast<char*>("string literal"); // sigh