tiff: Add PACKAGECONFIG for liblerc

tiff: Add PACKAGECONFIG for liblerc

tiff-native may pick up liblerc from the host using the .so
symlink, in which case it's added as a private requirement
in libtiff-4.pc. That in turn causes a build error in
gtk4-native in Yocto because Lerc.pc does not exist.

This was observed on Fedora 42 with gdk-pixbuf2-devel installed
on the host where libtiff is a dependency of gdk-pixbuf2 and
libtiff 4.7.0 is built with liblerc enabled.

Add a new PACKAGECONFIG setting for liblerc but keep it disabled
since liblerc does not exist in Yocto at the moment.

Signed-off-by: Zoltán Böszörményi <zboszor@gmail.com>
Backport-to: walnascar
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>