FTBFS on armfh - implicit-function-declaration

gram.c: In function ‘yyparse’:
gram.c:1531:16: error: implicit declaration of function ‘yylex’ [-Werror=implicit-function-declaration]
 1531 | yychar = yylex ();
      | ^~~~~
gram.y: In function ‘readin_config’:
gram.y:957:17: error: implicit declaration of function ‘yyset_in’ [-Werror=implicit-function-declaration]
  957 | yyset_in(in);
      | ^~~~~~~~
gram.y:965:17: error: implicit declaration of function ‘yylex_destroy’ [-Werror=implicit-function-declaration]
  965 | yylex_destroy();
      | ^~~~~~~~~~~~~

from Buildlog armhf [1].

Upstream's fix is at [2].

[1] https://launchpadlibrarian.net/724114432/buildlog_ubuntu-noble-armhf.radvd_1%3A2.19-1build1_BUILDING.txt.gz
[2] https://github.com/radvd-project/radvd/pull/196/commits