gdbserver-external: add missing MLPREFIX in dep on compilerlibs
DEPENDS/PROVIDES & PREFERRED_PROVIDER get mapped for mlprefix
automatically, but 'depends' flags do not, so the MLPREFIX has to be
explicit in such flags. This fixes the ability to build multilib images,
i.e. lib32-core-image-base.
Signed-off-by: Christopher Larson <chris_larson@mentor.com>