Fix problem with including libringbuffer/smp.h in make-dist

"make dist" currently produces a tarball that is missing
librinbuffer/smp.h, resulting in a build failure when an attempt to
compile the tarball is make.  This patch addresses the issue by properly
including the file in Makefile.am.

Signed-off-by: Gregory Haskins <gregory.haskins@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>