Lensfun has moved to sourceforge.net

The lensfun project has moved to sourceforge.net pending the close-down of
berlios.de by the end of April. This patch updates lensfun.mk accordingly.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Update lensfun's URL on website

Signed-off-by: Timothy Gu <timothygu99@gmail.com>