Handle block devices in main_file_stat, fixes "non-seekable" source bug for block devices

[adbjo]

Fixes the function for checking if the source is seekable (main_stat_file), so that block devices are correctly identified as seekable.

Currently, xdelta3 throws the following error when checking if block devices are seekable:

non-seekable source in decode: XD3_INTERNAL

Changes:

• Added inclusion of <sys/ioctl.h> and defined the BLKGETSIZE64 macro for block device size retrieval in xdelta3/xdelta3-main.h.
• Updated main_file_stat to detect block devices using S_ISBLK, retrieve their size with ioctl and BLKGETSIZE64, and set the file size accordingly.