Suppress the warning compiled with gcc-4.9

Fix the warning below:

  gatt_sr.c: In function 'gatt_sr_process_app_rsp':
  gatt_sr.c:175:27: warning: 'p_rsp' may be used uninitialized in this function [-Wmaybe-uninitialized]
                     p_rsp = (tGATTS_RSP *)GKI_getnext (p_rsp);
                           ^
  gatt_sr.c:125:23: note: 'p_rsp' was declared here
     tGATTS_RSP       *p_rsp;
                       ^

Change-Id: I894495ed7893e958f009778e629525b85d56b2f6