support GCC15

[cgzones]

GCC 15 starts warning about non NUL-terminated string literals:

chacha.c:44:31: error: initializer-string for array of ‘char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (17 chars into 16 available) [-Werror=unterminated-string-initialization]
   44 | static const char sigma[16] = "expand 32-byte k";
      |                               ^~~~~~~~~~~~~~~~~~