parse.y: fixed bison 3.0 warning

YACC  src/parse.y -> build/host/src/y.tab.c
src/parse.y:936.1-12: warning: deprecated directive, use ‘%pure-parser’
[-Wdeprecated]
%pure_parser
Merge pull request #1448 from cremno/bison-3.0-pure_parser-warning

fix bison 3.0 warning