Windows: Fix PRIuMAX definition.

Since GIT calls into Microsoft's MSVCRT.DLL, it must use the printf
format that this DLL uses for 64-bit integers, which is %I64u instead
of %llu.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

compat/mingw.h

@@ -161,3 +161,4 @@ int mingw_rename(const char*, const char*);
 #define has_dos_drive_prefix(path) (isalpha(*(path)) && (path)[1] == ':')
 #define is_dir_sep(c) ((c) == '/' || (c) == '\\')
 #define PATH_SEP ';'
+#define PRIuMAX "I64u"