Remove unnecessary 'extern's.

Change-Id: Iba2b3fb6ff88e504f1657b915120ae43d58a1e03
diff --git a/libc/include/errno.h b/libc/include/errno.h
index 2ff1369..8b2e8ed 100644
--- a/libc/include/errno.h
+++ b/libc/include/errno.h
@@ -41,7 +41,7 @@
 #endif
 
 /* internal function returning the address of the thread-specific errno */
-extern volatile int* __errno(void) __pure2;
+volatile int* __errno(void) __pure2;
 
 /* a macro expanding to the errno l-value */
 #define  errno   (*__errno())