Fix flockfile(3) and friends for stdin/stdout/stderr too.

stdin/stdout/stderr are special; their mutexes are initialized by
__sinit. There's no unit test for this, because __sinit has already
been called by the time the first unit test runs, but you could
reproduce this failure with a trivial main() that calls flockfile
or ftrylockfile on one of the standard streams before otherwise
using stdio.

Bug: 18208568
Change-Id: I28d232cf05a9f198a2bed61854d8047b23d2091d

libc/stdio/wcio.h

@@ -32,6 +32,10 @@
 #ifndef _WCIO_H_
 #define _WCIO_H_
 
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
 /* minimal requirement of SUSv2 */
 #define WCIO_UNGETWC_BUFSIZE 1
 
@@ -78,4 +82,6 @@ do {\
 #define WCIO_INIT(fp) \
 	memset(&(_EXT(fp)->_wcio), 0, sizeof(struct wchar_io_data))
 
+__END_DECLS
+
 #endif /*_WCIO_H_*/