dlmalloc: Fix a warning with clang-15

With clang-15 we now will get warnings such as:

warning: a function declaration without a prototype is deprecated in all
versions of C [-Wstrict-prototypes]

And it is easy enough to address this warning here, as we aren't
concerned with re-syncing with an upstream.

Signed-off-by: Tom Rini <[e-mail]>
Reviewed-by: Simon Glass <[e-mail]>

common/dlmalloc.c

···
 	return this;
 }
 
-void gcleanup ()
+void gcleanup (void)
 {
 	BOOL rval;
 	assert ( (head == NULL) || (head->base == (void*)gAddressBase));
···
 /* Utility to update current_mallinfo for malloc_stats and mallinfo() */
 
 #ifdef DEBUG
-static void malloc_update_mallinfo()
+static void malloc_update_mallinfo(void)
 {
   int i;
   mbinptr b;
···
 */
 
 #ifdef DEBUG
-void malloc_stats()
+void malloc_stats(void)
 {
   malloc_update_mallinfo();
   printf("max system bytes = %10u\n",
···
 */
 
 #ifdef DEBUG
-struct mallinfo mALLINFo()
+struct mallinfo mALLINFo(void)
 {
   malloc_update_mallinfo();
   return current_mallinfo;