VideoCommon/OnScreenDisplay: Take Message's std::string parameter by value

Allows callers to std::move strings into the functions (or automatically
assume the move constructor/move assignment operator for rvalue
references, potentially avoiding copies altogether.

Source/Core/Core/DSP/DSPHost.h

@@ -17,7 +17,7 @@ namespace DSP::Host
 {
 u8 ReadHostMemory(u32 addr);
 void WriteHostMemory(u8 value, u32 addr);
-void OSD_AddMessage(const std::string& str, u32 ms);
+void OSD_AddMessage(std::string str, u32 ms);
 bool OnThread();
 bool IsWiiHost();
 void InterruptRequest();