Fixed GCC 15 build with -Wunterminated-string-initialization.

Fixed GCC 15 build with -Wunterminated-string-initialization.

In file included from src/njs_main.h:48,
                 from src/njs_diyfp.c:12:
src/njs_string.h: In function ‘njs_string_encode’:
src/njs_string.h:229:36: error: initializer-string for array of ‘unsigned char’
	truncates NUL terminator but destination lacks ‘nonstring’ attribute (
	17 chars into 16 available) [-Werror=unterminated-string-initialization]
  229 |     static const u_char  hex[16] = "0123456789ABCDEF";