Disable mmap 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 3cf55950d0f6aa43eb5ad119bad1dbda69f75a54
  Author: Francis Deslauriers <francis.deslauriers@efficios.com>
  Date:   Fri Jun 5 11:38:14 2020 -0400

    syscalls: Make mmap()'s fields `prot` and `flags` enums

Change-Id: I033f42855c2967356b1e90cd89450eb5100a0f0b
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>