gr-digital: symbol_sync_ff_impl uses deprecated Boost header

[marcusmueller]

In file included from /usr/include/boost/config/header_deprecated.hpp:18,
                 from /usr/include/boost/math/common_factor_ct.hpp:14,
                 from /usr/include/boost/math/common_factor.hpp:13,
                 from /buildbot/gnuradio/gr-digital/lib/symbol_sync_ff_impl.cc:28:
/usr/include/boost/math/common_factor_ct.hpp:16:1: note: #pragma message: This header is deprecated. Use <boost/integer/common_factor_ct.hpp> instead.
   16 | BOOST_HEADER_DEPRECATED("<boost/integer/common_factor_ct.hpp>");
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/math/common_factor_rt.hpp:13:1: note: #pragma message: This header is deprecated. Use <boost/integer/common_factor_rt.hpp> instead.
   13 | BOOST_HEADER_DEPRECATED("<boost/integer/common_factor_rt.hpp>");
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/math/common_factor.hpp:16:1: note: #pragma message: This header is deprecated. Use <boost/integer/common_factor.hpp> instead.
   16 | BOOST_HEADER_DEPRECATED("<boost/integer/common_factor.hpp>");
      | ^~~~~~~~~~~~~~~~~~~~~~~

Boost deprecations are the wurst, since Boost seldom cleanly documents since when a new function is available from a certain file, so you end up in git-bisect, and then Ubuntu comes along and happily mixes versions of boost sublibraries. Often, only #ifdefs on Boost versions are a viable solution :/

A future-bound #ifdef __cplusplus >= 201703L using std::lcm might be cool when fixing this.