selftests: vDSO: quash clang omitted parameter warning in getrandom test

When building with clang, there's this warning:

vdso_test_getrandom.c:145:40: warning: omitting the parameter name in a function definition is a C23 extension [-Wc23-extensions]
  145 | static void *test_vdso_getrandom(void *)
      |                                        ^
vdso_test_getrandom.c:155:40: warning: omitting the parameter name in a function definition is a C23 extension [-Wc23-extensions]
  155 | static void *test_libc_getrandom(void *)
      |                                        ^
vdso_test_getrandom.c:165:43: warning: omitting the parameter name in a function definition is a C23 extension [-Wc23-extensions]
  165 | static void *test_syscall_getrandom(void *)

Add the named ctx parameter to quash it.

Reported-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>