build: consolidate to use one convenience lib

The original motivation for for splitting libbuxton-security out of
libbuxton-shared was to avoid using the duplicate Smack rule hashmap
from libbuxton back when bt-daemon linked with libbuxton.

Now that bt-daemon no longer links with libbuxton, and more functions
moved into shared/backend.c that should contain access checks, the
original reason for the split is no longer valid, making
libbuxton-security unnecessary.

This commit re-integrates the security lib in the common -shared
convenience library.