win32: only declare struct timespec when needed
win32: only declare struct timespec when needed

To make mingw cross compilation on Fedora 17 work.

[CC] src/common/autobuf.c
In file included from /usr/i686-w64-mingw32/sys-root/mingw/include/time.h:277:0,
                 from src/defs.h:54,
                 from src/common/autobuf.h:47,
                 from src/common/autobuf.c:42:
/usr/i686-w64-mingw32/sys-root/mingw/include/sys/timeb.h:90:8: error: redefinition of 'struct timespec'
In file included from src/defs.h:46:0,
                 from src/common/autobuf.h:47,
                 from src/common/autobuf.c:42:
./src/win32/sys/time.h:82:8: note: originally defined here

Signed-off-by: Ferry Huberts <66e11f9f5d965b69497c7e73d5e808f8b13c1218@pelagic.nl>

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