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/glue.h

@@ -32,6 +32,10 @@
  * SUCH DAMAGE.
  */
 
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
 /*
  * The first few FILEs are statically allocated; others are dynamically
  * allocated and linked in via this glue structure.
@@ -44,3 +48,5 @@ struct glue {
 
 /* This was referenced by a couple of different pieces of middleware and the Crystax NDK. */
 __LIBC64_HIDDEN__ extern struct glue __sglue;
+
+__END_DECLS