Fix bug in gdb.lookup_type

Fix bug in gdb.lookup_type

gdb.lookup_type accepts a 'block' argument, but in some cases does not
use it.  This can cause the wrong type to be returned.

This patch fixes the problem by simply passing the block through.  I
have no idea why it worked the way it did, and there weren't any tests
for the 'block' parameter.  (I didn't look at git blame out of fear
that it was my patch back in the day.)

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16942