s390/lib,string: fix strcat() inline asm constraint modifier

"dummy" is not only used as output but also as input. Therefore use
the correct "+" constraint modifier.

Fixes: 8cf23c8e

 ("s390/lib,string: get rid of register asm")
Reported-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>