package/libcap: needs headers >= 3.0

libcap requires XATTR_NAME_CAPS, which was introduced in Linux 2.6.24.

However, we have some toolchains, liek the CodeSourcery PowerPC 2011.03,
that is missing those defines, even though it uses headers from Linux
2.6.38.

Since there is no perfect way to avoid the situation, just require
headers >= 3.0 to build libcap.

Propagate the new dependency to packages that select libcap.

Fixes a lot of build failures, of which:
    http://autobuild.buildroot.net/results/e90/e909ca48ad2d1c85b19258f65e0c89a2813ac45e/
    http://autobuild.buildroot.net/results/23a/23aac9bb1cc48e5974bcf50256a16c67318f9ba2/
    http://autobuild.buildroot.net/results/597/597da58cb2fccb92d7883802c60b31264cacad08/
    http://autobuild.buildroot.net/results/538/538b23417c68f6c8c0c8c92dfc0e8ed314bac01b/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>