mimalloc: adjust for building inside Git

We want to compile mimalloc's source code as part of Git, rather than
requiring the code to be built as an external library: mimalloc uses a
CMake-based build, which is not necessarily easy to integrate into the
flavors of Git for Windows (which will be the main benefitting port).

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

compat/mimalloc/alloc.c

@@ -16,10 +16,6 @@ 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

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