build: Fix checkpoint_conf on mingw

CI flagged a failing mingw build, due to:
In file included from ../../src/conf/checkpoint_conf.c:24:
../gnulib/lib/configmake.h:8:17: error: expected identifier or '(' before string constant
    8 | #define DATADIR "/usr/i686-w64-mingw32/sys-root/mingw/share"
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As previously learned in commits bd205a90 and 976abdf6, gnulib's
configmake.h header does #define DATADIR "string...", while mingw's
<winsock2.h> expects to declare a type named DATADIR. As long as the
mingw system header is included first before configmake.h, the two
uses do not conflict, but until gnulib is patched to make configmake.h
automatically work around the issue, our immediate fix is the
workaround of rearranging our include order to insure no conflict.
Copy the paradigm used in domain_conf.c of using <unistd.h> to trigger
the indirect inclusion of <winsock2.h> on mingw.

Fixes: 1a4df34aSigned-off-by: NEric Blake <eblake@redhat.com>

src/conf/checkpoint_conf.c

@@ -21,6 +21,8 @@
 
 #include <config.h>
 
+#include <unistd.h>
+
 #include "configmake.h"
 #include "internal.h"
 #include "virbitmap.h"