Changed operations to allow to be compiled on VxWorks

While on Linux system stat_vfs() and fstat_vfs() are used to gain information about a mounted volume, VxWorks uses dirLib routines statfs() and fstatfs(), that are POSIX-compliant routines to gain information about a file system. 
This change includes the proper header for VxWorks to to allow the proper access for those functions.
My bad! You are right, as you pointed out stat.h does not need to be included and the correct is:

#     if !defined(__VXWORKS__)
#     include <sys/mount.h>
#     endif