Disable clone bitwise enum in default build

Disable clone bitwise enum in default build

Only generate the bitwise enumerations when
CONFIG_LTTNG_EXPERIMENTAL_BITWISE_ENUM is enabled, so the default build
does not generate traces which lead to warnings when viewed with
babeltrace 1.x and babeltrace 2 with default options.

Original commit:

  commit d775625e2ba4825b73b5897e7701ad6e2bdba115
  Author: Geneviève Bastien <gbastien@versatic.net>
  Date:   Wed Apr 1 14:31:49 2020 -0400

    syscalls: Make clone()'s `flags` field a 2 enum struct.

Change-Id: Ia67b5fc93b932b8b76330b4965753a43401c1514
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>