Temporarily revert "mimalloc: adjust for building inside Git"

In preparation for upgrading to a newer mimalloc version.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mimalloc/alloc.c

@@ -16,6 +16,10 @@ terms of the MIT license. A copy of the license can be found in the file
 #include <string.h>      // memset, strlen (for mi_strdup)
 #include <stdlib.h>      // malloc, abort
 
+#define MI_IN_ALLOC_C
+#include "alloc-override.c"
+#undef MI_IN_ALLOC_C
+
 // ------------------------------------------------------
 // Allocation
 // ------------------------------------------------------

compat/mimalloc/mimalloc.h

@@ -95,8 +95,7 @@ terms of the MIT license. A copy of the license can be found in the file
 // Includes
 // ------------------------------------------------------
 
-#include "git-compat-util.h"
-
+#include <stddef.h>     // size_t
 #include <stdbool.h>    // bool
 #include <stdint.h>     // INTPTR_MAX