crimson: workaround an ICE of GCC

crimson: workaround an ICE of GCC

this change works around the FTBFS on arm64:

/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/crimson/common/config_proxy.h:74:13:
internal compiler error: in tsubst_decomp_names, at cp/pt.c:16537
             for (auto& [obs, keys] : rev_obs) {
             ^~~
Please submit a full bug report,
with preprocessed source if appropriate.

it seems that this issue is a dup of
https://bugzilla.redhat.com/show_bug.cgi?id=1639019 .

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