bug fix: missing common.h include before using PYBIND11_SIMPLE_GIL_MANAGEMENT

For the tests in the github CI this does not matter, because
`PYBIND11_SIMPLE_GIL_MANAGEMENT` is always defined from the command line,
but when monkey-patching common.h locally, it matters.

Author: rwgk

include/pybind11/detail/internals.h

@@ -9,6 +9,8 @@
 
 #pragma once
 
+#include "common.h"
+
 #if defined(WITH_THREAD) && defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
 #    include "../gil.h"
 #endif