www/osrm-backend: Fix build: 'deadline_timer' is deprecated: Use system_timer

In file included from /wrkdirs/usr/ports/www/osrm-backend/work/osrm-backend-0ae940c31566df2aec0ecd679c84c11ede9831ef/src/tools/r
outed.cpp:1:
In file included from /wrkdirs/usr/ports/www/osrm-backend/work/osrm-backend-0ae940c31566df2aec0ecd679c84c11ede9831ef/include/ser
ver/server.hpp:4:
/wrkdirs/usr/ports/www/osrm-backend/work/osrm-backend-0ae940c31566df2aec0ecd679c84c11ede9831ef/include/server/connection.hpp:53:
18: error: 'deadline_timer' is deprecated: Use system_timer [-Werror,-Wdeprecated-declarations]

53 |     boost::asio::deadline_timer timer;
   |                  ^

/usr/local/include/boost/asio/deadline_timer.hpp:36:1: note: 'deadline_timer' has been explicitly marked deprecated here

36 | BOOST_ASIO_DEPRECATED_MSG("Use system_timer")
   | ^

/usr/local/include/boost/asio/detail/config.hpp:1484:47: note: expanded from macro 'BOOST_ASIO_DEPRECATED_MSG'
1484 | # define BOOST_ASIO_DEPRECATED_MSG(msg) deprecated(msg)

|                                               ^

1 error generated.

• Add -Wno-deprecated-declarations to the CXXFLAGS.
• While here fix warning from portclippy.

PR: 297799
Sponsored by: UNIS Labs
MFH: 2026Q3