hw/nxagent/Screen.h: Provide prototype for nxagentAdjustRandRXinerama(). Avoid implicit-declaration compiler warning.

 Amends the following compiler warning:

 ```
 Screen.c:3716:5: warning: implicit declaration of function ‘nxagentAdjustRandRXinerama’ [-Wimplicit-function-declaration]
      nxagentAdjustRandRXinerama(pScreen);
      ^
 ```