gdb/block: make find_iterator_compunit_symtab a method of block_iterator

I noticed some code in bkscm_print_block_syms_progress_smob peeking a
bit in the implementation details of block_iterator, and doing
essentially the same thing as the existing static function
find_iterator_compunit_symtab.

Change find_iterator_compunit_symtab to become
block_iterator::compunit_symtab, and use it in
bkscm_print_block_syms_progress_smob.

Change-Id: I344a2155a2edb5d278e98684c71f1ee161e8d1d4
Approved-By: Tom Tromey <tom@tromey.com>