Amends the following the compiler warnings:

 ```
 Extensions.c: In function ‘nxagentRandRInitSizes’:
 Extensions.c:159:16: warning: unused variable ‘pRandRScrPriv’ [-Wunused-variable]
    rrScrPrivPtr pRandRScrPriv = rrGetScrPriv(pScreen);
                 ^
 Extensions.c: In function ‘nxagentRandRSetConfig’:
 Extensions.c:263:16: warning: unused variable ‘pRandRScrPriv’ [-Wunused-variable]
    rrScrPrivPtr pRandRScrPriv;
                 ^
 Extensions.c: In function ‘nxagentRandRScreenSetSize’:
 Extensions.c:322:16: warning: unused variable ‘pRandRScrPriv’ [-Wunused-variable]
    rrScrPrivPtr pRandRScrPriv;
                 ^
 ```