libcap: Add optional support for libattr capabilities

libcap can optionally link to libattr to support extra
file capabilities. Link to this library and pull it as
dependency if BR2_PACKAGE_ATTR is selected.

[Peter: use LIBCAP_ prefix on variable]
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>