tracing/ftrace: make boot tracer select the sched_switch tracer

Impact: build fix

If the boot tracer is selected but not the sched_switch,
there will be a build failure:

 kernel/built-in.o: In function `boot_trace_init':
 trace_boot.c:(.text+0x5ee38): undefined reference to `sched_switch_trace'
 kernel/built-in.o: In function `disable_boot_trace':
 (.text+0x5eee1): undefined reference to `tracing_stop_cmdline_record'
 kernel/built-in.o: In function `enable_boot_trace':
 (.text+0x5ef11): undefined reference to `tracing_start_cmdline_record'

This patch fixes it.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/Kconfig

@@ -123,6 +123,7 @@ config BOOT_TRACER
 	bool "Trace boot initcalls"
 	depends on DEBUG_KERNEL
 	select TRACING
+	select CONTEXT_SWITCH_TRACER
 	help
 	  This tracer helps developers to optimize boot times: it records
 	  the timings of the initcalls and traces key events and the identity