khash: silence -Wunused-function for delta-islands

khash: silence -Wunused-function for delta-islands

showing the following when compiled with latest clang (OpenBSD, Fedors
and macOS):

delta-islands.c:23:1: warning: unused function 'kh_destroy_str'
      [-Wunused-function]
delta-islands.c:23:1: warning: unused function 'kh_clear_str'
      [-Wunused-function]
delta-islands.c:23:1: warning: unused function 'kh_del_str' [-Wunused-function]

Reported-by: René Scharfe <redacted>
Suggested-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Carlo Marcelo Arenas Belón <redacted>
Signed-off-by: Junio C Hamano <redacted>