macro: fix lxcfs_{error,debug,v} build error when __VA_ARGS__ is empty

macro: fix lxcfs_{error,debug,v} build error when __VA_ARGS__ is empty

Originally, the compiler complained:
  macro.h:7:25: error: expected expression before ')' token
      __func__, __VA_ARGS__);

The reason is that GCC wouldn't abandon `,` when `__VA_ARGS__` is empty.
For emaple:
  #define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__)
  eprintf("success!\n", );
       → fprintf(stderr, "success!\n", );

According to GCC doc, it's okay when adding `##` before `__VA_ARGS__`:
  #define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__)
  eprintf ("success!\n")
       → fprintf(stderr, "success!\n");

Signed-off-by: river <river@vvl.me>