[S390] exec_protect: Fix incorrect extern declarations.

sys_sigreturn and sys_rt_sigreturn don't take any arguments. So luckily
this resulted only in unneeded instead of incorrect code.
But still this clearly shows why one should not put extern declarations
in C files (will be fixed with a larger sparse patch).

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