parser.y update for deprecated %pure-parser

[mckellygit]

Hi,
Thank you for libxkbcommon.
I get this when building -
../src/xkbcomp/parser.y:88.1-12: warning: deprecated directive: ‘%pure-parser’, use ‘%define api.pure’ [-Wdeprecated]
and changed parser.y to use newer method.
Not sure if changing causes more harm than good or if we can version this somehow.
I am using bison 3.5.1
thx

[bluetech]

Yes this is annoying, the reason that I haven't changed it so far is that we also support byacc, which didn't support @define api.pure. BUT, I just checked again and

2023-05-16  Thomas E. Dickey  <[email protected]>
	* reader.c:
	recognize bison's "%define api.pure" as an alternative to "%pure-parser"

and a quick test shows that it works. So we can merge this, thanks!