ead: fix compilation with GCC 15

ead: fix compilation with GCC 15

GCC 15 defaults to C23, where an empty parameter list "()" means
"(void)" instead of an unspecified argument list. This broke two spots
in the bundled tinysrp sources:

  - The interrupt() signal handler in t_truerand.c was declared with
    "()", so its type became "void (*)(void)" and no longer matched the
    "void (*)(int)" expected by signal():

      t_truerand.c:100:32: error: passing argument 2 of 'signal' from
      incompatible pointer type [-Wincompatible-pointer-types]

    Give it the proper signal handler signature instead.

  - The pre-ANSI fallback in t_defines.h declared strchr(), strrchr()
    and strtok() with "()". ead.c is compiled without -DHAVE_CONFIG_H,
    so STDC_HEADERS is undefined and that legacy branch is taken, where
    the zero-argument prototypes now conflict with the declarations in
    the standard headers:

      tinysrp/t_defines.h:90:30: error: conflicting types for 'strtok';
      have 'char *(void)'

    Drop the dead K&R branch and include <stdlib.h> and <string.h>
    unconditionally; the library's own objects already take this path
    via config.h.

Assisted-by: Claude:claude-opus-4-8
Link: https://github.com/openwrt/openwrt/pull/23955
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>