kernel: generic: add missing 4.19 config symbol

kernel: generic: add missing 4.19 config symbol

Fixes following kernel build issue on ath79 with CONFIG_KERNEL_FTRACE=y
enabled:

 Tracers (FTRACE) [Y/n/?] y
   Kernel Function Tracer (FUNCTION_TRACER) [Y/n/?] y
     Kernel Function Graph Tracer (FUNCTION_GRAPH_TRACER) [Y/n/?] y
   Enable trace events for preempt and irq disable/enable (PREEMPTIRQ_EVENTS) [N/y/?] (NEW)
   ...
   Preempt / IRQ disable delay thread to test latency tracers (PREEMPTIRQ_DELAY_TEST) [N/m/?] (NEW)

Signed-off-by: Petr Štetiar <ynezz@true.cz>