libcap2: fix build with and without libpam

libcap2: fix build with and without libpam

* Add libpam to depends if DISTRO_FEATURES contains pam.

* Fixes the following compile error, when libpam-dev is installed
  on the host, but not built for the target:

pam_cap.c:19:34: error: security/pam_modules.h: No such file or directory
pam_cap.c:20:34: error: security/_pam_macros.h: No such file or directory

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
CC: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
CC: Otavio Salvador <otavio@ossystems.com.br>
CC: Tom Rini <tom_rini@mentor.com>
CC: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>