libethumb: Fix automagic dependencies on libedbus and libexif

libethumb will determine during configuration phase whether to build the
optional libexif and libedbus modules. It will enable this modules if
libedbus or libexif are present on the target system. Therefore, we need
to add these packages as optional dependencies to libethumb.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>