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

indra/llui/tests/llurlentry_stub.cpp

@@ -193,8 +193,8 @@ LLFontGL* LLFontGL::getFontDefault()
 	return NULL; 
 }
 
-char* _PREHASH_AgentData = "AgentData";
-char* _PREHASH_AgentID = "AgentID";
+char* _PREHASH_AgentData = const_cast<char*>("AgentData");
+char* _PREHASH_AgentID   = const_cast<char*>("AgentID");
 
 LLHost LLHost::invalid(INVALID_PORT,INVALID_HOST_IP_ADDRESS);