applypatch: Compile libimgpatch for target and host.

update_engine need it for the new IMGDIFF operation.

Also removed __unused in ApplyImagePatch() as I got error building it
for the host, and I think it's dangerous not checking the size of the
input.

Test: mma
Bug: 26628339

Change-Id: I22d4cd55c2c3f87697d6afdf10e8106fef7d1a9c