FreeType comes with two licenses from which you can choose the one which fits your needs best.
The FreeType License (FTL) is the most commonly used one. It is a BSD-style license with a credit clause and thus compatible with the GNU Public License (GPL) version 3, but not with the GPL version 2.
The GNU General Public License (GPL), version 2. Use it for all projects which use the GPLv2 also, or which need a license compatible to the GPLv2.