fix: take a const parameter in 'aagerror'

fix: take a const parameter in 'aagerror'

This function does not modify the pointed to data of its argument. However, the
immediate problem this change is addressing is that Bison 3.8 now emits a
prototype for this function:¹

  To comply with the latest POSIX standard, in Yacc compatibility mode
  (options `-y`/`--yacc`) Bison now generates prototypes for yyerror and
  yylex.  In some situations, this is breaking compatibility: if the user
  has already declared these functions but with some differences (e.g., to
  declare them as static, or to use specific attributes), the generated
  parser will fail to compile.  To disable these prototypes, #define yyerror
  (to `yyerror`), and likewise for yylex.

Because the generated prototype takes a const parameter,² these two prototypes
now conflict causing a compilation error. Rather than the work around suggested
by the Bison notes, this change simply re-aligns `aagerror` with the Bison
default.

Related to #2127.

¹ https://github.com/akimd/bison/blob/master/NEWS
² https://www.gnu.org/software/bison/manual/html_node/Error-Reporting-Function.html