bpo-35081: Move Py_BUILD_CORE code to internal/mem.h (GH-10249)

* Add #include "internal/mem.h" to C files using
  _PyMem_SetDefaultAllocator().
* Include/internal/mem.h now requires Py_BUILD_CORE to be defined.
diff --git a/Modules/main.c b/Modules/main.c
index 455178a..6a8aa05 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -2,6 +2,7 @@
 
 #include "Python.h"
 #include "osdefs.h"
+#include "internal/mem.h"
 #include "internal/pygetopt.h"
 #include "internal/pystate.h"