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/plat/common/platform.h b/include/plat/common/platform.h
index e614cdb..aa181c8 100644
--- a/include/plat/common/platform.h
+++ b/include/plat/common/platform.h
@@ -386,12 +386,14 @@
 
 unsigned int plat_get_aff_count(unsigned int, unsigned long);
 unsigned int plat_get_aff_state(unsigned int, unsigned long);
+
 #else /* __ENABLE_PLAT_COMPAT__ */
 /*
  * The below function enable Trusted Firmware components like SPDs which
  * haven't migrated to the new platform API to compile on platforms which
  * have the compatibility layer disabled.
  */
+unsigned int platform_core_pos_helper(unsigned long mpidr);
 unsigned int platform_get_core_pos(unsigned long mpidr) __deprecated;
 
 #endif /* __ENABLE_PLAT_COMPAT__ */