Fails to build on Arch

[vlovich]

The declaration of ais_charset.c generates a warning on GCC 15 which fails if you build with Werror=format-security (the default on Arch).

ais_charset.c:3:24: error: initializer-string for array of ‘char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (65 chars into 64 available) [-Werror=unterminated-string-initialization]
    3 | char ais_charset[64] = "@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_ !\"#$%&'()*+,-./0123456789:;<=>?";
      |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  SoapySDR support: yes

This should either be ais_charset[65] or ais_charset[64] __attribute__((nonstring)) (not sure about the target compilers to be supported in the code - the attribute may need to be hidden behind a macro).