libcap: fix licensing fields

libcap: fix licensing fields

The recipe LICENSE field should reflect the license of all of the sources,
so include the PAM module licenses too.

Don't use conditionals in LIC_FILES_CHKSUM so that the configuration
does not cause differing checksums to be used.

Explicitly set the license on libcap and libcap-dev to reflect the
contents:
- libcap is BSD|GPLv2, and also BSD|LGPLv2+ if PAM is enabled.
- libcap-dev is just the library, so BSD|GPLv2.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>