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>

target/linux/generic/config-4.19

@@ -3831,6 +3831,8 @@ CONFIG_PPP_MULTILINK=y
 CONFIG_PREEMPT_NONE=y
 # CONFIG_PREEMPT_TRACER is not set
 # CONFIG_PREEMPT_VOLUNTARY is not set
+# CONFIG_PREEMPTIRQ_EVENTS is not set
+# CONFIG_PREEMPTIRQ_DELAY_TEST is not set
 CONFIG_PREVENT_FIRMWARE_BUILD=y
 CONFIG_PRINTK=y
 CONFIG_PRINTK_NMI=y