fixup! mimalloc: adjust for building inside Git

Part of the series that drops the vendored `mimalloc` from this fork;
the rationale (no measurable speedup over the platform allocator on
any of Windows, macOS, or Linux) is in the second commit of the
series and the PR description. The `fixup!` subject is so the next
rebase against an upstream Git that already lacks the target commit
autosquashes this revert into it, dropping the original cleanly.

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

Author: dscho

compat/mimalloc/alloc.c

@@ -17,6 +17,7 @@ terms of the MIT license. A copy of the license can be found in the file
 #include <stdlib.h>      // malloc, abort
 
 #define MI_IN_ALLOC_C
+#include "alloc-override.c"
 #include "free.c"
 #undef MI_IN_ALLOC_C
 

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 "compat/posix.h"
-
+#include <stddef.h>     // size_t
 #include <stdbool.h>    // bool
 #include <stdint.h>     // INTPTR_MAX