build: work around super-old readline.h

build: work around super-old readline.h

This patch shuts up the following warning of clang
on Mac OS X:

  virsh.c:2761:22: error: assigning to 'char *' from 'const char [6]' discards qualifiers
      [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
      rl_readline_name = "virsh";
                       ^ ~~~~~~~

The warning happens because rl_readline_name on Mac OS X comes
from an old readline header that still uses 'char *', while it
is 'const char *' in readline 4.2 (April 2001) and newer.

Tested on Mac OS X 10.8.5 (clang-500.2.75) and Fedora 19 (gcc 4.8.1).

Signed-off-by: Ryota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: Eric Blake <eblake@redhat.com>