tiffdec: use toff_t for exif ifd offset
tiffdec: use toff_t for exif ifd offset

versions < 4.0.0 used uint32 interchangeably, but with 4.0.0 toff_t
became 64-bit and TIFFTAG_EXIFIFD began returning it rather than
TIFF_LONG/uint32.

Change-Id: I42492bd24613a884c7496e7bfc0c5d892758bce9

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