hurd: Fix includability of <hurd/signal.h> in all standards

* bits/sigaction.h: Add include guard.
* sysdeps/unix/sysv/linux/alpha/bits/sigaction.h: Likewise.
* sysdeps/unix/sysv/linux/bits/sigaction.h: Likewise.
* sysdeps/unix/sysv/linux/hppa/bits/sigaction.h: Likewise.
* sysdeps/unix/sysv/linux/ia64/bits/sigaction.h: Likewise.
* sysdeps/unix/sysv/linux/mips/bits/sigaction.h: Likewise.
* sysdeps/unix/sysv/linux/s390/bits/sigaction.h: Likewise.
* sysdeps/unix/sysv/linux/sparc/bits/sigaction.h: Likewise.
* sysdeps/unix/sysv/linux/tile/bits/sigaction.h: Likewise.
* hurd/hurd/signal.h: Include <bits/sigaction.h>.