maint: Improve use of configmake.h on mingw

Gnulib has added a patch that allows configmake.h to be included
without causing build failures on mingw if <winsock2.h> is later
included (whether directly, or indirectly such as through gnulib's
<unistd.h>).

This reverts commit fed58d83c6 ("build:
Fix checkpoint_conf on mingw"), now that we don't have to worry about
header inclusion ordering issues.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/conf/checkpoint_conf.c

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