lavfi/Makefile: Don't compile lavfutils.c and lswsutils.c if they are not needed

They are only needed by removelogo.
This fixes a link failure in shared builds when compiling with
removelogo disabled but Lavf/SwS enabled

Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>