Fix compiler warning.
Fix compiler warning.

* subversion/libsvn_diff/diff_file.c
  (svn_diff_file_output_unified3): 'char **c' -> 'const char **c'

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion