buildroot

Add libxpm as a dependency of ace_of_penguins

ace_of_penguins fails to build when xlib_libXpm is not selected. Add
it has a dependency. However, contrary to normal dependencies to
libraries (where we use 'select') we use 'depends on' here, otherwise
the X.org server will be get selected automatically without the user
noticing.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>