cmake: link denc-mod-rgw against Boost::filesystem

cmake: link denc-mod-rgw against Boost::filesystem

to address the runtime link failure.

this change is not cherry-picked from main branch. as, in main branch,
the Boost::filesystem linkage is pulled in by rgw_common, which was
changed to a static library in 43d10b9e44ca50700e9076a47f2c38b360d1d632.
but this change is not included in pacific. so rgw added the linkage
via rgw_libs CMake variable. unfortunately, the lexical scope of this
variable does not not include tools/ceph-dencoder/CMakeLists.txt, so
we have to add this linkage manually here.

Signed-off-by: Tim Serong <tserong@suse.com>
Signed-off-by: Kefu Chai <tchaikov@gmail.com>