community/shadow: add libcap-dev to enable HAVE_SYS_CAPABILITY_H
Add libcap-dev
to makedepends
for enabling HAVE_SYS_CAPABILITY_H
flag.
https://github.com/shadow-maint/shadow/blob/4.8.1/libmisc/idmapping.c#L150-L185
Required for https://github.com/moby/buildkit/issues/2038