util: Fix warning with const qualifier from bsearch

  CC       src/util.o
src/util.c: In function ‘codepoint_lookup’:
src/util.c:2643:16: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
 2643 |         result = bsearch(key, table, len, sizeof(struct codepoint),
      |                ^