package/mono: fixing monodoc leaking path

This patch fixes a leak in host compiler path for monodoc.
This bug is solved upstream but the fix is not included
in the latest stable release.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>