tools_common.h: (msvs) avoid redefining off_t

in some configurations MSVS will define _off_t / off_t in wchar.h; the
former is used locally while the latter is for compatibility. this
change overrides off_t as in the past and sets _OFF_T_DEFINED to prevent
a clash in types.

Change-Id: I9b0e6db586a0a2729b545d93edfc56570d2fcf97
1 file changed