fix regression in s390x SO_PEERSEC definition

analogous to commit efda534b212f713fe2b92a62b06e45f656b763ce for
powerpc. commit 587f5a53bc3a68d80b239ba515d583df690a96df moved the
definition of SO_PEERSEC to bits/socket.h for archs where the SO_*
macros differ.

arch/s390x/bits/socket.h

@@ -38,6 +38,7 @@ struct cmsghdr {
 #define SO_RCVTIMEO     20
 #define SO_SNDTIMEO     21
 #define SO_ACCEPTCONN   30
+#define SO_PEERSEC      31
 #define SO_SNDBUFFORCE  32
 #define SO_RCVBUFFORCE  33
 #define SO_PROTOCOL     38