Fix build of tests with mingw

very, very partial backport of 2f795d8f to maint/v0.21 branch

without this include, O_RDWR and O_CREAT are undeclared when building tests/path/win32.c with mingw

src/path.h

@@ -8,6 +8,7 @@
 #define INCLUDE_path_h__
 
 #include "common.h"
+#include "posix.h"
 #include "buffer.h"
 #include "vector.h"