selftests/powerpc: Fix compile error on tlbie_test due to newer gcc

Newer versions of GCC (>= 9) demand that the size of the string to be
copied must be explicitly smaller than the size of the destination.
Thus, the NULL char has to be taken into account on strncpy.

This will avoid the following compiling error:

  tlbie_test.c: In function 'main':
  tlbie_test.c:639:4: error: 'strncpy' specified bound 100 equals destination size
      strncpy(logdir, optarg, LOGDIR_NAME_SIZE);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Signed-off-by: Desnes A. Nunes do Rosario <desnesn@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191003211010.9711-1-desnesn@linux.ibm.com