systemd: workaround compilation failures against old glibc

Building against older libc/kernel headers can fail, because our glue
code for systemd has issues. Fix them by forward declaring "struct
statx" and by disabling parts of "socket-util.c".

shared/systemd/sd-adapt-shared/nm-sd-adapt-shared.h

@@ -140,6 +140,10 @@ NM_PRAGMA_WARNING_DISABLE("-Wdeclaration-after-statement")
 
 /*****************************************************************************/
 
+struct statx;
+
+/*****************************************************************************/
+
 static inline pid_t
 raw_getpid(void)
 {

shared/systemd/src/basic/socket-util.c

@@ -1217,6 +1217,7 @@ ssize_t recvmsg_safe(int sockfd, struct msghdr *msg, int flags) {
         return n;
 }
 
+#if 0 /* NM_IGNORED */
 int socket_get_family(int fd, int *ret) {
         int af;
         socklen_t sl = sizeof(af);
@@ -1393,3 +1394,4 @@ int socket_set_transparent(int fd, int af, bool b) {
                 return -EAFNOSUPPORT;
         }
 }
+#endif /* NM_IGNORED */