timesync,test: explicitly include sys/timex.h when struct timex is used

struct timex is defined by sys/timex.h -> bits/timex.h.
Glibc includes the header in time.h, but let's explicitly include it
when the struct is used.

Similar to 4f18ff2e29, but for sys/timex.h.

src/test/test-sizeof.c

@@ -7,6 +7,7 @@
 #include <string.h>
 #include <sys/resource.h>
 #include <sys/socket.h>
+#include <sys/timex.h>
 
 #define __STDC_WANT_IEC_60559_TYPES_EXT__
 #include <float.h>

src/timesync/timesyncd-manager.c

@@ -5,6 +5,7 @@
 #include <netinet/in.h>
 #include <netinet/ip.h>
 #include <resolv.h>
+#include <sys/timex.h>
 #include <unistd.h>
 
 #include "sd-bus.h"