gdbserver: include hostio.h in hostio-errno.c

gdbserver: include hostio.h in hostio-errno.c

... so that the definition of hostio_last_error_from_errno in hostio-errno.c
sees the declaration in hostio.h.

Fix this error:

      CXX    hostio-errno.o
    /home/smarchi/src/binutils-gdb/gdb/gdbserver/hostio-errno.c: In function ‘void hostio_last_error_from_errno(char*)’:
    /home/smarchi/src/binutils-gdb/gdb/gdbserver/hostio-errno.c:28:1: error: no previous declaration for ‘void hostio_last_error_from_errno(char*)’ [-Werror=missing-declarations]
     hostio_last_error_from_errno (char *buf)
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

gdb/gdbserver/ChangeLog:

	* hostio-errno.c: Include hostio.h.

Change-Id: I056308fd4ce12810d0a1b826c423bd0c7eeb8944