crda: Makefile: fix .so compilation line with some compilers

Write the CC rule such that it only tries to produce a single output
file (the .so). When including the .h files in the compiler invocation,
this suggests we should be producing pre-compiled headers too, which
doesn't make sense in this context, and Clang happens not to like.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>