curlx_base64_encode: use uint8_t* for input

Change `inputbuff` parameter from `const char *` to `const uint8_t *` to
reflect the binary nature of the input bytes. Half the code was casting
unsigned char to signed already in calling.

Closes #19722

lib/http.c

@@ -367,7 +367,8 @@ static CURLcode http_output_basic(struct Curl_easy *data, bool proxy)
   if(!out)
     return CURLE_OUT_OF_MEMORY;
 
-  result = curlx_base64_encode(out, strlen(out), &authorization, &size);
+  result = curlx_base64_encode((uint8_t *)out, strlen(out),
+                               &authorization, &size);
   if(result)
     goto fail;