sane-backends.pc.in: Don't embed LDFLAGS into libs.private
If you build sane-backends in an OS buildsystem, LDFLAGS usually contains buildsystem related flags, which are irrelevant outside from the buildsystem and can cause problems.
LDFLAGS in libs.private adds flags which are not required for building sane-backends and can create confusion for people who want to build sane-backends locally.