Add some defines to build succesfully on FreeBSD

Tested on FreeBSD 13.2 only.  FreeBSD is more similar to Mac OS X in
system-level library support than it is to Linux.

src/include/86box/plat_dir.h

@@ -66,6 +66,9 @@ extern void           seekdir(DIR *, long);
 extern int            closedir(DIR *);
 
 #    define rewinddir(dirp) seekdir(dirp, 0L)
+#elif defined(__FreeBSD__)
+/* FreeBSD uses dirent.h instead of sys/dir.h */
+#    include <dirent.h>
 #else
 /* On linux and macOS, use the standard functions and types */
 #    include <sys/dir.h>