test/test-sizeof: Include sys/timex.h for struct timex

Fixes

../git/src/test/test-sizeof.c:64:41: error: incomplete definition of type 'struct timex'
   64 |         check(typeof(((struct timex *)0)->freq), SIZEOF_TIMEX_MEMBER);
      |                      ~~~~~~~~~~~~~~~~~~~^

src/test/test-sizeof.c

@@ -5,6 +5,7 @@
 #include <string.h>
 #include <sys/resource.h>
 #include <sys/socket.h>
+#include <sys/timex.h>
 #include <sys/types.h>
 
 #define __STDC_WANT_IEC_60559_TYPES_EXT__