libffi: fix staging headers cleanup

The LIBFFI_MOVE_STAGING_HEADERS function is responsible for moving
around libffi headers to a standard location. Once this is done, it
removes the no longer used directory, but does so in $(TARGET_DIR) and
not $(STAGING_DIR). This directory is already cleaned up in
$(TARGET_DIR) in the LIBFFI_REMOVE_TARGET_HEADERS post-install target
hook.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>