package/osm2pgsql: needs std::future

package/osm2pgsql: needs std::future

osm2pgsql needs std::future since its addition in commit
46c42042867e672eb21b386eb5e29747c30e80ca to avoid the following build
failure:

In file included from /home/buildroot/autobuild/run/instance-3/output-1/build/osm2pgsql-1.4.2/src/db-copy.cpp:12:0:
/home/buildroot/autobuild/run/instance-3/output-1/build/osm2pgsql-1.4.2/src/db-copy.hpp:218:24: error: field 'barrier' has incomplete type
     std::promise<void> barrier;
                        ^
/home/buildroot/autobuild/run/instance-3/output-1/build/osm2pgsql-1.4.2/src/db-copy.hpp: In constructor 'db_cmd_sync_t::db_cmd_sync_t(std::promise<void>&&)':
/home/buildroot/autobuild/run/instance-3/output-1/build/osm2pgsql-1.4.2/src/db-copy.hpp:221:37: error: class 'db_cmd_sync_t' does not have any field named 'barrier'
     : db_cmd_t(db_cmd_t::Cmd_sync), barrier(std::move(b))
                                     ^
In file included from /home/buildroot/autobuild/run/instance-3/output-1/build/osm2pgsql-1.4.2/src/middle.hpp:18:0,
                 from /home/buildroot/autobuild/run/instance-3/output-1/build/osm2pgsql-1.4.2/src/dependency-manager.cpp:11:
/home/buildroot/autobuild/run/instance-3/output-1/build/osm2pgsql-1.4.2/src/thread-pool.hpp: In member function 'void thread_pool_t::check_for_exceptions()':
/home/buildroot/autobuild/run/instance-3/output-1/build/osm2pgsql-1.4.2/src/thread-pool.hpp:49:19: error: invalid use of incomplete type 'class std::future<void>'
             result.get();
                   ^

Fixes:
 - http://autobuild.buildroot.org/results/69528232ceba128a2e60aa778bb0943fdefc2ed1

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>