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>