[CMAKE] Fix FindBerkeleyDB for FreeBSD

Summary:
The files are located under /usr/local/<include|lib>/db5/ on FreeBSD 12.
Using PATH_SUFFIXES allow to search with the db5 directory in the
path.

Test Plan:
On FreeBSD (tested on FreeBSD 12):

cmake -GNinja -DBUILD_BITCOIN_QT=OFF ..

Check that cmake finds the library.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4709