fix(zynqmp): resolve misra R8.4 warnings

MISRA Violation: MISRA-C:2012 R.8.4
- Function definition does not have a visible prototype.

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@xilinx.com>
Change-Id: I50a2c1adf2e099217770ac665f135302f990b162
diff --git a/plat/xilinx/zynqmp/sip_svc_setup.c b/plat/xilinx/zynqmp/sip_svc_setup.c
index 4a6095c..4ce9b8a 100644
--- a/plat/xilinx/zynqmp/sip_svc_setup.c
+++ b/plat/xilinx/zynqmp/sip_svc_setup.c
@@ -53,7 +53,7 @@
  * Handler for all SiP SMC calls. Handles standard SIP requests
  * and calls PM SMC handler if the call is for a PM-API function.
  */
-uintptr_t sip_svc_smc_handler(uint32_t smc_fid,
+static uintptr_t sip_svc_smc_handler(uint32_t smc_fid,
 			      u_register_t x1,
 			      u_register_t x2,
 			      u_register_t x3,