Fix compilation on Ubuntu Linux

Without this change:

```
$ make
...
demo/decode_ft8.c:347:31: error: ‘CLOCK_REALTIME’ undeclared (first use in this function)
  347 |                 clock_gettime(CLOCK_REALTIME, &spec);
```

demo/decode_ft8.c

@@ -1,3 +1,5 @@
+#define _POSIX_C_SOURCE 199309L
+
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>