Remove unnecessary 'extern's.

Change-Id: Iba2b3fb6ff88e504f1657b915120ae43d58a1e03
diff --git a/libc/include/android/legacy_signal_inlines.h b/libc/include/android/legacy_signal_inlines.h
index edc6259..8ba7894 100644
--- a/libc/include/android/legacy_signal_inlines.h
+++ b/libc/include/android/legacy_signal_inlines.h
@@ -37,7 +37,7 @@
 
 __BEGIN_DECLS
 
-extern sighandler_t bsd_signal(int signum, sighandler_t handler) __REMOVED_IN(21);
+sighandler_t bsd_signal(int signum, sighandler_t handler) __REMOVED_IN(21);
 
 #if __ANDROID_API__ < 21