mk: set library dependencies in shared object file

mk: set library dependencies in shared object file

Some .so libraries needs to be linked with external libraries.  For that the
LDLIBS and EXTRA_LDFLAGS variables should be present on the link line when
those .so files are created.  PMD Makefile is responsible for filling the
LDLIBS variable with the link to the external library it needs.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>