clang: suppress a warning

Set missing default value.

    tsv.c:105:16: warning: variable 'encoding' is used uninitialized whenever 'if'
          condition is false [-Wsometimes-uninitialized]
        } else if (strncasecmp(encoding_name, "KOI8-R", strlen("KOI8-R")) == 0 ||
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    tsv.c:114:10: note: uninitialized use occurs here
      return encoding;
             ^~~~~~~~
    tsv.c:105:12: note: remove the 'if' if its condition is always true
        } else if (strncasecmp(encoding_name, "KOI8-R", strlen("KOI8-R")) == 0 ||
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    tsv.c:79:3: note: variable 'encoding' is declared here
      grn_encoding encoding;
      ^