Remove several warnings reported with W=1

Improved support for W=1 compilation flag by solving missing-prototypes
and old-style-definition warnings.

The libraries are compiling with warnings (which turn into errors with
the Werror flag).

Outside of libraries, some warnings cannot be fixed without heavy
structural changes.

Change-Id: I1668cf99123ac4195c2a6a1d48945f7a64c67f16
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
diff --git a/plat/arm/css/sgi/sgi_topology.c b/plat/arm/css/sgi/sgi_topology.c
index f9dbdef..7aa9e40 100644
--- a/plat/arm/css/sgi/sgi_topology.c
+++ b/plat/arm/css/sgi/sgi_topology.c
@@ -5,6 +5,7 @@
  */
 
 #include <plat/arm/common/plat_arm.h>
+#include <plat/common/platform.h>
 
 #include <sgi_variant.h>