build: e_gadget_loader is using dlsym

but is not explicitly linking against it. Previously this was not
discovered due to a wrong flag in elementaries pkgconfig. However - the
new .pc file of elementary does not contain dl anymore (as no library
_needs_ to link against it when using elm). So we need to link this here
explicitly

Differential Revision: https://phab.enlightenment.org/D7150