Remove extern keyword from function declarations

Function declarations implicitly have external linkage so do not
need the extern keyword.

Change-Id: Ia0549786796d8bf5956487e8996450a0b3d79f32
diff --git a/bl2/bl2_private.h b/bl2/bl2_private.h
index 2a66d42..022d1e9 100644
--- a/bl2/bl2_private.h
+++ b/bl2/bl2_private.h
@@ -34,6 +34,6 @@
 /******************************************
  * Function prototypes
  *****************************************/
-extern void bl2_arch_setup(void);
+void bl2_arch_setup(void);
 
 #endif /* __BL2_PRIVATE_H__ */