fts: Fix GCC compile error

GCC does not support the blocks extension, contrary to clang with which
it is automatically enabled by our infrastructure (see
'lib/libc/gen/Makefile.inc') when compiling 'fts.c'. The alternate code
(blocks extension not supported/enabled) tried to dereference a 'void *'
pointer (field 'fts_compar_b' of 'FTS') to access field 'isa' of the
block mocked by 'block_abi.h'. Fix this by casting the pointer to the
block type.

Reviewed by: jhb, des
Fixes: f0ac5e919f3f ("fts: Add blocks support.")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50068