lib: Fix ordering of RB Tree

lib: Fix ordering of RB Tree

So the *bsd implementations of RB Tree's for older
platforms use a macro implementation.  New platforms
have converted to a function implementation that uses
a different calling parameter list.  So when
we attempt to build FRR on older *bsd implementations
the macro's and functions do not interact too well.

As a workaround put the openbsd-tree.h #include
inside of zebra.h at a point before the particular
platforms version is included.  Since we use
the same #if guard for the header we should be ok.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>