dmclock: pick up change to use specified C++ settings if any

see https://github.com/ceph/dmclock/pull/73

so dmclock won't add `-std=c++11` to CXX_FLAGS even if the parent project already
have `-std=c++17` in CXX_FLAGS.

Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/src/dmclock b/src/dmclock
index a86236a1960066c418c1a5bee0d5a41d25dd30c3..1c2a7d5491268c19ed5b4b38d9ae075420b4a0d5 160000 (submodule)
--- a/src/dmclock
+++ b/src/dmclock
@@ -1 +1 @@
-Subproject commit a86236a1960066c418c1a5bee0d5a41d25dd30c3
+Subproject commit 1c2a7d5491268c19ed5b4b38d9ae075420b4a0d5