[S390] ptrace: no extern declarations for userspace

/include/asm/ptrace.h:275: extern's make no sense in userspace
/include/asm/ptrace.h:279: extern's make no sense in userspace
/include/asm/ptrace.h:280: extern's make no sense in userspace

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