c/syscalls.c: add missing <time.h> icnlude

[trofi]

Without the change the build fails for me as:

c/syscalls.c: In function 'sleep_until':
c/syscalls.c:711:18: error: implicit declaration of function 'time' [-Wimplicit-function-declaration]
  711 |     time_t now = time(0);
      |                  ^~~~
c/syscalls.c:41:1: note: 'time' is defined in header '<time.h>'; this is probably fixable by adding '#include <time.h>'
   40 | #include "syscalls.h"
  +++ |+#include <time.h>
   41 |