openssh: bump version

From the announcement:

This release fixes a security bug:

 * sshd(8): fix a memory corruption problem triggered during rekeying
   when an AES-GCM cipher is selected. Full details of the vulnerability
   are available at: http://www.openssh.com/txt/gcmrekey.adv

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/openssh/openssh.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-OPENSSH_VERSION = 6.3p1
+OPENSSH_VERSION = 6.4p1
 OPENSSH_SITE = http://ftp.openbsd.org/pub/OpenBSD/OpenSSH/portable
 OPENSSH_CONF_ENV = LD="$(TARGET_CC)" LDFLAGS="$(TARGET_CFLAGS)"
 OPENSSH_CONF_OPT = --disable-lastlog --disable-utmp \