build failure due to implicit declaration

Job #4826727578 failed for 182696dd:

curl_ntlm_core.c:508:17: error: implicit declaration of function 'tolower' [-Werror=implicit-function-declaration]
  508 |         pw[i] = tolower(password[i+33]);
      |                 ^~~~~~~
curl_ntlm_core.c:131:1: note: include '<ctype.h>' or provide a declaration of 'tolower'
  130 | #include "memdebug.h"
  +++ |+#include <ctype.h>