GCC 15.0 enables -Wunterminated-string-initialization when using
'-Wall -Wextra', which gets used when I run './configure'. This patch
silences the warnings:

fns.c: In function ‘hexbuf_digest’:
fns.c:6019:40: warning: initializer-string for array of ‘char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (17 chars into 16 available) [-Wunterminated-string-initialization]
 6019 |       static char const hexdigit[16] = "0123456789abcdef";
      |                                        ^~~~~~~~~~~~~~~~~~

Collin