Fix MISRA rule 8.4 in common code

Rule 8.4: A compatible declaration shall be visible when
          an object or function with external linkage is defined.

Change-Id: I26e042cb251a6f9590afa1340fdac73e42f23979
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
diff --git a/services/std_svc/std_svc_setup.c b/services/std_svc/std_svc_setup.c
index ffc3471..eae078e 100644
--- a/services/std_svc/std_svc_setup.c
+++ b/services/std_svc/std_svc_setup.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014-2017, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2014-2018, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -58,7 +58,7 @@
  * Top-level Standard Service SMC handler. This handler will in turn dispatch
  * calls to PSCI SMC handler
  */
-uintptr_t std_svc_smc_handler(uint32_t smc_fid,
+static uintptr_t std_svc_smc_handler(uint32_t smc_fid,
 			     u_register_t x1,
 			     u_register_t x2,
 			     u_register_t x3,