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)