Increase output buffer size

Increase output buffer size

This fixes the compiler error:

../../src/main.c: In function ‘packet_cb’:
../../src/main.c:426:52: error: ‘__builtin___snprintf_chk’ output may be truncated before the last format character [-Werror=format-truncation=]
  426 |   snprintf(buf, sizeof(buf), "External interrupt %u",
      |                                                    ^
In file included from /usr/include/stdio.h:867,
                 from ../../src/main.c:24:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:67:10: note: ‘__builtin___snprintf_chk’ output between 21 and 25 bytes into a destination of size 24
   67 |   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   68 |        __bos (__s), __fmt, __va_arg_pack ());
      |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Marc Schink <dev@zapb.de>