Makefile.am: Fix make distcheck

The attribute test is called tests/check_attributes, not
tests/check_attribute.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Introcude <riemann/riemann-client.h>

Already mentioned by the API docs, introduce this header that includes
all others (and <errno.h>), so that everything one will need is
available with a single include.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>