[ffigen] Update LICENSE (#457) Follow Protobuf's practice of including a trailer in the LICENSE file.

diff --git a/pkgs/ffigen/LICENSE b/pkgs/ffigen/LICENSE
index 467a982..c7e642d 100644
--- a/pkgs/ffigen/LICENSE
+++ b/pkgs/ffigen/LICENSE

@@ -25,3 +25,6 @@
 THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+Code generated by FFIgen is owned by the owner of the input file used
+when generating it.