databases/pg.el: Fix build of full, canna, nox and wayland flavors

Since 0.34 pg.el requires peg.el. Peg.el was added to master branch of
Emacs repository on March but isn't included in release version of
Emacs yet. As a result build of this port succeeds with devel_full and
devel_nox flavors but fails with others. So fix build by adding
devel/peg.el as dependency.

Fixes: 0d43ebf72f2a "databases/pg.el: Update to 0.34"