databases/mdbtools: Workaround breakage from llvm15

parser.c:1035:9: error: variable 'yynerrs' set but not used [-Werror,-Wunused-but-set-variable]

int yynerrs = 0;

The parser code is created at build time. llvm15 complains about the not used variable.
Avoiding llvm15 is just a workaround, until a better solution is around.

Reported by: pkg-fallout