Remove unnecessary 'extern's.

Change-Id: Iba2b3fb6ff88e504f1657b915120ae43d58a1e03
diff --git a/libc/include/sys/prctl.h b/libc/include/sys/prctl.h
index d96b8b6..742ed57 100644
--- a/libc/include/sys/prctl.h
+++ b/libc/include/sys/prctl.h
@@ -35,7 +35,7 @@
 
 __BEGIN_DECLS
 
-extern int prctl(int option, ...);
+int prctl(int option, ...);
 
 __END_DECLS