build: use _GL_ATTRIBUTE_NONSTRING attribute to avoid new warnings

build: use _GL_ATTRIBUTE_NONSTRING attribute to avoid new warnings

Avoid warnings like this from GCC 15:
src/basenc.c:1139:20: error: initializer-string for array of 'char'
truncates NUL terminator but destination lacks 'nonstring' attribute
(9 chars into 8 available) [-Werror=unterminated-string-initialization]
* src/basenc.c (z85_encoding, do_decode): Mark two more variables as
non-terminated.