include: Move fprintf_function to disas/

The previous commits have eliminated fprintf_function outside
disassemblers, simplifying code and cleaning up the ugly type-punning
fprintf_function seems to attract.  Move fprintf_function to
include/disas/dis-asm.h to reduce the temptation to abuse it.

I considered renaming it to fprintf_ftype (reverting that part of
commit 6e2d864e, v0.14.0) to get us closer to binutils, but I
figure the fork is too distant to make this worthwhile.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190417191805.28198-18-armbru@redhat.com>