Advertise --libdir in configure --help output

The configure script allows you to supply a libdir via --libdir but was
not advertising this in --help.
Signed-off-by: NDoug Goldstein <cardoe@cardoe.com>
CC: qemu-trivial@nongnu.org
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

configure

@@ -1050,6 +1050,7 @@ echo "  --mandir=PATH            install man pages in PATH"
 echo "  --datadir=PATH           install firmware in PATH$confsuffix"
 echo "  --docdir=PATH            install documentation in PATH$confsuffix"
 echo "  --bindir=PATH            install binaries in PATH"
+echo "  --libdir=PATH            install libraries in PATH"
 echo "  --sysconfdir=PATH        install config in PATH$confsuffix"
 echo "  --localstatedir=PATH     install local state in PATH"
 echo "  --with-confsuffix=SUFFIX suffix for QEMU data inside datadir and sysconfdir [$confsuffix]"