Support Ctrl-C to cancel rendering with headless builds

The original intention from [0] was to force background-mode when built
WITH_HEADLESS or WITH_PYTHON_MODULE, with the else clause setting the
signal handler for Ctrl-C. Since WITH_PYTHON_MODULE now disables all
signal handlers this check no longer makes sense.

[0]: 9d9c05a101

source/creator/creator_signals.c

@@ -244,11 +244,9 @@ void main_signal_setup_background(void)
   /* for all platforms, even windows has it! */
   BLI_assert(G.background);
 
-#  if !defined(WITH_HEADLESS)
   /* Support pressing `Ctrl-C` to close Blender in background-mode.
    * Useful to be able to cancel a render operation. */
   signal(SIGINT, sig_handle_blender_esc);
-#  endif
 }
 
 void main_signal_setup_fpe(void)