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/include/lib/el3_runtime/cpu_data.h b/include/lib/el3_runtime/cpu_data.h
index 3f48de5..15d34eb 100644
--- a/include/lib/el3_runtime/cpu_data.h
+++ b/include/lib/el3_runtime/cpu_data.h
@@ -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
  */
@@ -102,6 +102,8 @@
 #endif
 } __aligned(CACHE_WRITEBACK_GRANULE) cpu_data_t;
 
+extern cpu_data_t percpu_data[PLATFORM_CORE_COUNT];
+
 #if CRASH_REPORTING
 /* verify assembler offsets match data structures */
 CASSERT(CPU_DATA_CRASH_BUF_OFFSET == __builtin_offsetof