src/services/yaac.y: use '%define api.pure'
Instead of the deprecated '%pure-parser', c.f.: https://lists.gnu.org/archive/html/bug-gnulib/2019-07/msg00068.html
This fixes a compilation warning: services/yacc.y:83.1-12: warning: deprecated directive: ‘%pure-parser’, use ‘%define api.pure’ [-Wdeprecated]