windows/mpconfigport: Provide off_t definition for MSVC port

For MSVC off_t is defined in sys/types.h but according to the comment
earlier in mpconfigport.h this cannot be included directly.
So just make off_t the same as mp_off_t.
This fixes the build for MSVC with MICROPY_STREAMS_POSIX_API
enabled because stream.h uses off_t.

ports/windows/mpconfigport.h

@@ -232,6 +232,7 @@ typedef __int64                     ssize_t;
 #define SSIZE_MAX                   _I32_MAX
 typedef int                         ssize_t;
 #endif
+typedef mp_off_t                    off_t;
 
 
 // Put static/global variables in sections with a known name