misc: fix -Walloc-size warnings

GCC 14 introduces a new -Walloc-size included in -Wextra which gives:

```
src/tool_operate.c: In function ‘add_per_transfer’:
src/tool_operate.c:213:5: warning: allocation of insufficient size ‘1’ for type ‘struct per_transfer’ with size ‘480’ [-Walloc-size]
  213 |   p = calloc(sizeof(struct per_transfer), 1);
      |     ^
src/var.c: In function ‘addvariable’:
src/var.c:361:5: warning: allocation of insufficient size ‘1’ for type ‘struct var’ with size ‘32’ [-Walloc-size]
  361 |   p = calloc(sizeof(struct var), 1);
      |     ^
```

The calloc prototype is:
```
void *calloc(size_t nmemb, size_t size);
    ```

So, just swap the number of members and size arguments to match the
prototype, as we're initialising 1 struct of size `sizeof(struct
...)`. GCC then sees we're not doing anything wrong.

Closes #12292

src/tool_operate.c

@@ -211,7 +211,7 @@ static curl_off_t all_pers;
 static CURLcode add_per_transfer(struct per_transfer **per)
 {
   struct per_transfer *p;
-  p = calloc(sizeof(struct per_transfer), 1);
+  p = calloc(1, sizeof(struct per_transfer));
   if(!p)
     return CURLE_OUT_OF_MEMORY;
   if(!transfers)

src/var.c

@@ -358,7 +358,7 @@ static ParameterError addvariable(struct GlobalConfig *global,
   if(check)
     notef(global, "Overwriting variable '%s'", check->name);
 
-  p = calloc(sizeof(struct var), 1);
+  p = calloc(1, sizeof(struct var));
   if(!p)
     return PARAM_NO_MEM;