core: mem/tlsf - return unsigned long for function giving available memory

- aling it with mem api prototype

mem/tlsf_malloc.c

@@ -1138,10 +1138,10 @@ void tlsf_meminfo(tlsf_t pool, struct mem_info *info)
 	info->total_size = control->total_size;
 }
 
-size_t tlsf_available(tlsf_t pool)
+unsigned long tlsf_available(tlsf_t pool)
 {
 	control_t* control = tlsf_cast(control_t*, pool);
-	return control->total_size - control->real_used;
+	return (unsigned long)(control->total_size - control->real_used);
 }
 
 void tlsf_status(tlsf_t pool)

mem/tlsf_malloc.h

@@ -79,7 +79,7 @@ int tlsf_check_pool(pool_t pool);
 void tlsf_meminfo(tlsf_t pool, struct mem_info *info);
 void tlsf_status(tlsf_t pool);
 void tlsf_sums(tlsf_t pool);
-size_t tlsf_available(tlsf_t pool);
+unsigned long tlsf_available(tlsf_t pool);
 void tlsf_mod_get_stats(tlsf_t pool, void **root);
 void tlsf_mod_free_stats(void *root);