xpp: Do not typedef bool on RHEL 5.2 or later.

Without digging into the specifics, it looks like Red Hat Linux 5.9
removed the hex_asc definition that was previously used to determine
if the bool definition was backported.

We can simply use the RHEL_RELEASE_CODE now since we do not support any
releases before the 5 series now.

Internal-Issue-ID: DAHLIN-312
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Reported-By: Vladimir Mikhelson