package/libcap: Fix build error with kernel headers < 3.6

package/libcap: Fix build error with kernel headers < 3.6

Fixes
http://autobuild.buildroot.net/results/cce/cceb1ccacec36fb7ef41bb7cdb13b3014813b599/

XATTR_NAME_CAPS appears in kernel headers since 3.7:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/log/include/uapi/linux/xattr.h?id=v3.7

Before it was an internal define of the kernel:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/include/linux/xattr.h?id=607ca46e97a1b6594b29647d98a32d545c24bdff

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>