mk: fix extra options when linking lib through compiler

When using a linker option not known by the compiler like -rpath,
the library linkage was failing.
It is fixed by prefixing the option with -Wl, as it is done in other
makefiles.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>