src/udev/udev-event.c must #include <sys/wait.h>

Fixes the following build failure with musl:
../git/src/udev/udev-event.c: In function 'spawn_wait':
../git/src/udev/udev-event.c:600:53: error: 'WEXITED' undeclared (first use in this function); did you mean 'WIFEXITED'?
         r = sd_event_add_child(e, NULL, spawn->pid, WEXITED, on_spawn_sigchld, spawn);
                                                     ^~~~~~~

This looks like a bug in udev-event.c that could also have broken
the compilation after some future glibc header reshuffle.

src/udev/udev-event.c

@@ -7,6 +7,7 @@
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
 #include "sd-event.h"