xfsprogs: Fix mismatched return type of filesize()

xfsprogs: Fix mismatched return type of filesize()

The function filesize() was declared with a return type of 'long' but
defined with 'off_t'. This mismatch caused build issues due to type
incompatibility.

This commit updates the declaration to match the definition, ensuring
consistency and preventing potential compilation errors.

Fixes: 73fb78e5ee8 ("mkfs: support copying in large or sparse files")
Signed-off-by: Pavel Reichl <preichl@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cem@kernel.org>
Fixes: 73fb78e5ee8 ("mkfs: support copying in large or sparse files")
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>