[3.13] gh-122584: Import mimalloc headers in a C++ context (GH-122587) (#123035)

gh-122584: Import mimalloc headers in a C++ context (GH-122587)
(cherry picked from commit 1dad23e)

Co-authored-by: Michael Droettboom <[email protected]>

Author: miss-islington

Include/internal/pycore_mimalloc.h

@@ -36,9 +36,18 @@ typedef enum {
 #  define MI_TSAN 1
 #endif
 
+#ifdef __cplusplus
+extern "C++" {
+#endif
+
 #include "mimalloc/mimalloc.h"
 #include "mimalloc/mimalloc/types.h"
 #include "mimalloc/mimalloc/internal.h"
+
+#ifdef __cplusplus
+}
+#endif
+
 #endif
 
 #ifdef Py_GIL_DISABLED