package/boost: disable logs with riscv32

boost logs can't be built with riscv32 because it unconditionally uses
__NR_futex:

libs/log/src/event.cpp: In member function 'void boost::log::v2_mt_posix::aux::futex_based_event::wait()':
libs/log/src/event.cpp:38:29: error: '__NR_futex' was not declared in this scope
   38 | #define BOOST_LOG_SYS_FUTEX __NR_futex
      |                             ^~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/8c8135fd7c0517c66c9b3975c494da6d7934cc1b



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>