mbview_plot.c in 5.8.2beta20 misses time.h include

I failed to compile 5.8.2beta20 in current Ubuntu 24.04.2 (Kernel 6.11.0-21) with the following error:

```
MB-System-5.8.2beta20/src/mbview/mbview_plot.c:857:15: error: ‘CLOCK_REALTIME’ undeclared (first use in this function) 
  857 | clock_gettime(CLOCK_REALTIME, &starttime);
```

To me adding 
`#include <time.h>`
to mbview_plot.c as first include fixed the issue.