Warning on yacc build

See comment on f0165614 (comment 456921)

After this commit - still getting the following warning

Executing (target): make 
  YACC     src/file/keydbcfg-parser.c
  LEX      src/file/keydbcfg-lexer.c
/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-13.0-devel/build/libaacs-0.11.1/src/file/keydbcfg-parser.y:126.1-7: warning: POSIX Yacc does not support %define [-Wyacc]
  126 | %define api.pure
      | ^~~~~~~
updating src/file/keydbcfg-parser.h