feature tests: do not re-include <stdio.h>

iffe --man documents that stdio.h is automatically pre-included for
all feature tests. Including it in the test code is not needed.

You'd think it shouldn't do any harm, but on a Gentoo i386 system,
this include turned out to be the cause of a mysterious 'unknown
type: off64_t' error while compiling the output{ ... }end block in
features/pty. I'm not going to bother with further tracing the
cause of that -- there is some hackery with off64_t defines in the
AST headers that probably has something to do with it.

src/cmd/builtin/features/pty,
src/lib/libast/features/float,
src/lib/libast/features/sfio,
src/lib/libast/features/stdio:
- Remove '#include <stdio.h>' from output{ ... }end blocks.

src/cmd/builtin/features/pty

@@ -13,7 +13,6 @@ lib	openpty,_getpty,ptsname -lutil
 lib	grantpt,unlockpt,posix_openpt stdlib.h
 
 tst - output{
-	#include	<stdio.h>
 	#include	<sys/types.h>
 	#include	<sys/stat.h>
 	#if _lib_ptsname && _npt_ptsname