Remove unnecessary 'extern's.

Change-Id: Iba2b3fb6ff88e504f1657b915120ae43d58a1e03
diff --git a/libc/include/sys/vfs.h b/libc/include/sys/vfs.h
index bf094dc..1231eb1 100644
--- a/libc/include/sys/vfs.h
+++ b/libc/include/sys/vfs.h
@@ -137,10 +137,10 @@
 #define XENIX_SUPER_MAGIC     0x012FF7B4
 #define XFS_SUPER_MAGIC       0x58465342
 
-extern int statfs(const char* _Nonnull, struct statfs* _Nonnull);
-extern int statfs64(const char* _Nonnull, struct statfs64* _Nonnull) __INTRODUCED_IN(21);
-extern int fstatfs(int, struct statfs* _Nonnull);
-extern int fstatfs64(int, struct statfs64* _Nonnull) __INTRODUCED_IN(21);
+int statfs(const char* _Nonnull, struct statfs* _Nonnull);
+int statfs64(const char* _Nonnull, struct statfs64* _Nonnull) __INTRODUCED_IN(21);
+int fstatfs(int, struct statfs* _Nonnull);
+int fstatfs64(int, struct statfs64* _Nonnull) __INTRODUCED_IN(21);
 
 __END_DECLS