LibC: Make the address argument of malloc_size a pointer to const

We don't mutate the pointed-to memory, so let's be const correct.

Fixes building the `mimalloc` library that's optionally used by the mold
linker (note that it isn't enabled yet as I haven't tested it).

Userland/Libraries/LibC/malloc.cpp

@@ -500,7 +500,7 @@ void* calloc(size_t count, size_t size)
  return ptr;
 }
 
-size_t malloc_size(void* ptr)
+size_t malloc_size(void const* ptr)
 {
  MemoryAuditingSuppressor suppressor;
  if (!ptr)

Userland/Libraries/LibC/stdlib.h

@@ -20,7 +20,7 @@ __attribute__((noreturn)) void _abort(void);
 
 __attribute__((malloc)) __attribute__((alloc_size(1))) void* malloc(size_t);
 __attribute__((malloc)) __attribute__((alloc_size(1, 2))) void* calloc(size_t nmemb, size_t);
-size_t malloc_size(void*);
+size_t malloc_size(void const*);
 size_t malloc_good_size(size_t);
 void serenity_dump_malloc_stats(void);
 void free(void*);