lib/rbtree.h: Add missing NAEMON_END_DECL

An unclosed 'extern "C" {' is a good way to screw up C++ builds
against C headers. This adds that missing end-of-declarations
thing so C++ knows that its own files are name-mangled.

Thus the build should work again.

Signed-off-by: Andreas Ericsson <ae@op5.se>