meson: find ldap instead of link flags

The current meson adds link flags for libldap directly. This makes that
meson doesn't error out until compilation (e.g., report missing ldap.h).

This commit adds the ldap dependency explicitly and makes it required.

Tested:
1. CI works
2. bitbake works

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I7513a1a825b28c85afc51248e5b93d69a34e82f4
1 file changed