dtoolutil: Set system malloc tag to mimalloc when enabled

dtool/src/dtoolutil/pandaSystem.cxx

@@ -58,6 +58,8 @@ PandaSystem() :
   set_system_tag("system", "malloc", "dlmalloc");
 #elif defined(USE_MEMORY_PTMALLOC2)
   set_system_tag("system", "malloc", "ptmalloc2");
+#elif defined(USE_MEMORY_MIMALLOC)
+  set_system_tag("system", "malloc", "mimalloc");
 #else
   set_system_tag("system", "malloc", "malloc");
 #endif