libsigc: Don't copy devel documentation onto target

The libsigc++ developer documentation was being copied onto the target.
Add a clean-up to fix that.

Signed-off-by: Charles Manning <cdhmanning@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>