s390/dis: move disassembler function prototypes to proper header file

Now that the in-kernel disassembler has an own header file move the
disassembler related function prototypes to that header file.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>