fixup??? mimalloc: adjust for building inside Git

There is not actually any need to include all of `git-compat-util.h`,
now that the relevant parts were moved to `compat/posix.h`.

It also fixes a compile problem with USE_MIMALLOC on Linux: the
`skip_iprefix()` function needs to have the `tolower()` function defined
already, but when included from `reftable/basics.c` that is not the case
due to the peculiar #include chain. The symptom looks like this:

  In file included from compat/mimalloc/mimalloc.h:98,                                                                                     from ./compat/posix.h:180,
                   from reftable/system.h:14,
                   from reftable/basics.h:16,
                   from reftable/basics.c:10:
  ./git-compat-util.h: In function ‘skip_iprefix’:
  ./git-compat-util.h:907:18: error: implicit declaration of function ‘tolower’ [-Werror=implicit-function-declaration]
    907 |         } while (tolower(*str++) == tolower(*prefix++));
        |                  ^~~~~~~
  ./git-compat-util.h:468:1: note: include ‘<ctype.h>’ or provide a declaration of ‘tolower’
    467 | #include "wrapper.h"
    +++ |+#include <ctype.h>
    468 |

This is easily fixed by including just `compat/posix.h` and not
`git-compat-util.h` (since the latter #includes the former, this
is essentially a nice scope reduction).

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

Author: dscho

compat/mimalloc/mimalloc.h

@@ -95,7 +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 "compat/posix.h"
 
 #include <stdbool.h>    // bool
 #include <stdint.h>     // INTPTR_MAX