scanner.l: fix a few implicit conversion warnings

scanner.l: fix a few implicit conversion warnings

  LEX      src/liboconfig/scanner.c
  CC       src/liboconfig/liboconfig_la-scanner.lo
src/liboconfig/scanner.l:113:12: warning: implicit conversion loses integer precision: 'unsigned long' to 'int' [-Wshorten-64-to-32]
        int len = strlen (yytext);
            ~~~   ^~~~~~~~~~~~~~~
src/liboconfig/scanner.l:129:12: warning: implicit conversion loses integer precision: 'unsigned long' to 'int' [-Wshorten-64-to-32]
        int len = strlen (yytext);
            ~~~   ^~~~~~~~~~~~~~~