missing.h: Define IFA_F_NOPREFIXROUTE

Fixes second systemd compilation problem against Linux 3.12 uapi
headers.

IFA_F_NOPREFIXROUTE is a usual #define appeared in Linux 3.14, so
AC_CHECK_DECLS is not necessary.

src/shared/missing.h

@@ -863,6 +863,10 @@ static inline int setns(int fd, int nstype) {
 #define IFA_FLAGS 8
 #endif
 
+#ifndef IFA_F_NOPREFIXROUTE
+#define IFA_F_NOPREFIXROUTE 0x200
+#endif
+
 #ifndef MAX_AUDIT_MESSAGE_LENGTH
 #define MAX_AUDIT_MESSAGE_LENGTH 8970
 #endif