suppress warnings from using Bison extensions in lib/expr/exparse.y

suppress warnings from using Bison extensions in lib/expr/exparse.y

This is the equivalent of 8c099fc032074a0bb5961d67ef4df05ddca7eba4 but for a
different parser. Surprisingly exparse.y was already using Bison extensions, so
this actually suppresses two warnings:

  bison -y -dtv ../../lib/expr/exparse.y
  ../../lib/expr/exparse.y:131.1-7: warning: POSIX Yacc does not support %binary [-Wyacc]
   %binary       <op>    EQ      NE
   ^~~~~~~
  ../../lib/expr/exparse.y:132.1-7: warning: POSIX Yacc does not support %binary [-Wyacc]
   %binary       <op>    '<'     '>'     LE      GE
   ^~~~~~~

Related to #1806.