compat: fix compiler warning in nla_get_s64()

compat: fix compiler warning in nla_get_s64()

This fixes the following warning:

compat/include/linux/compat-3.7.h: In function ‘nla_get_s64’:
compat/include/linux/compat-3.7.h:191:2: warning: passing argument 2 of ‘nla_memcpy’ discards ‘const’ qualifier from pointer target type [enabled by default]
In file included from include/net/genetlink.h:5:0,
                 from compat/include/linux/compat-2.6.32.h:12,
                 from compat/include/linux/compat-2.6.29.h:13,
                 from compat/include/linux/compat-2.6.h:50,
                 from <command-line>:0:

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>