kselftest/arm64: Fix missing printf() argument in gcs/gcs-stress.c

Compiling the child_cleanup() function results in:

gcs-stress.c: In function ‘child_cleanup’:
gcs-stress.c:266:75: warning: format ‘%d’ expects a matching ‘int’ argument [-Wformat=]
  266 |                                 ksft_print_msg("%s: Exited due to signal %d\n",
      |                                                                          ~^
      |                                                                           |
      |                                                                           int

Add the missing child->exit_signal argument.

Fixes: 05e6cfff ("kselftest/arm64: Add a GCS stress test")
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>