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/local.h

@@ -41,6 +41,8 @@
 #include "wcio.h"
 #include "fileext.h"
 
+__BEGIN_DECLS
+
 /*
  * Android <= KitKat had getc/putc macros in <stdio.h> that referred
  * to __srget/__swbuf, so those symbols need to be public for LP32
@@ -137,3 +139,5 @@ extern int __sfvwrite(FILE *, struct __suio *);
 wint_t __fputwc_unlock(wchar_t wc, FILE *fp);
 
 #pragma GCC visibility pop
+
+__END_DECLS