misc/bits/select2.h: Clearly separate declaration from definitions

The __fdelt_chk declaration needs to be available so that
libc_hidden_proto can be used while not redefining __FD_ELT.
Thus, misc/bits/select-decl.h is created to hold the corresponding
prototypes.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

misc/bits/select-decl.h

@@ -0,0 +1,31 @@
+/* Checking routines for select functions. Declaration only.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _BITS_SELECT_DECL_H
+#define _BITS_SELECT_DECL_H 1
+
+#ifndef _SYS_SELECT_H
+# error "Never include <bits/select-decl.h> directly; use <sys/select.h> instead."
+#endif
+
+/* Helper functions to issue warnings and errors when needed.  */
+extern long int __fdelt_chk (long int __d);
+extern long int __fdelt_warn (long int __d)
+  __warnattr ("bit outside of fd_set selected");
+
+#endif