libselinux: Fix ARM build failure

Avoid this warning (which is in turn an error via -Werror):
````
avc_internal.c: In function 'avc_netlink_receive':
avc_internal.c:105:25: error: cast increases required alignment of target type [-Werror=cast-align]
 struct nlmsghdr *nlh = (struct nlmsghdr *)buf;
                        ^
````

The code allocates abuffer with "__attribute__ ((aligned))",
then passes it via a 'char*' parameter, which is then finally cast,
causing the warning. So the code is ok but compiler is not smart
enough to see it.

It seems that -Wcast-align is a no-op on x86, so this shows up on ARM
only.