Don't give warnings in compat

There are lots of warnings in here due to things like modules being
imported that, in some versions of GHC, aren't used. Thus we don't
give any warnings in here, and therefore validating with -Werror won't
make the build fail.

An alternative would be to do
SRC_HC_OPTS := $(filter-out -Werror,$(SRC_HC_OPTS))
but if warnings are expected then there is little point in spewing them
out anyway.

On the other hand, there aren't any warnings for me (GHC 6.6 on Linux/amd64),
so perhaps it would be worth fixing them instead.