unigbrk: Avoid gcc warnings.

unigbrk: Avoid gcc warnings.

* lib/unigbrk/u16-grapheme-breaks.c (u16_grapheme_breaks): Remove
unused variable.
* lib/unigbrk/u16-grapheme-prev.c (u16_grapheme_prev): Likewise.
* lib/unigbrk/u8-grapheme-prev.c (u8_grapheme_prev): Likewise.
* tests/unigbrk/test-u16-grapheme-breaks.c (main): Likewise.
* tests/unigbrk/test-u32-grapheme-breaks.c (main): Likewise.
* tests/unigbrk/test-u8-grapheme-breaks.c (test_u8_grapheme_breaks):
Change type of first argument to 'const char *'.
(main): Remove unused variable.
* tests/unigbrk/test-u8-grapheme-next.c (test_u8_grapheme_next): Change
type of first argument to 'const char *'.
* tests/unigbrk/test-u8-grapheme-prev.c (test_u8_grapheme_prev):
Likewise.
(main): Change type of variable 's'.
* tests/unigbrk/test-uc-is-grapheme-break.c (main): Cast column number
to 'int'.