tip: Build failure with !CONFIG_TRACE - undefined reference to `trace_clock_global'

["Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>]

With tip (commit - 58e70a841b20d5d80aebdd8274ab60c0c933470f) I get a build
error when CONFIG_TRACE is not set. 

arch/x86/kernel/built-in.o: In function `ds_switch_to':
(.text+0x86ee): undefined reference to `trace_clock_global'
arch/x86/kernel/built-in.o: In function `ds_switch_to':
(.text+0x8743): undefined reference to `trace_clock_global'
make[1]: *** [.tmp_vmlinux1] Error 1
make: *** [sub-make] Error 2

Looks like it is coming from the change
commit 15879d042164650b93d83281ad5f87ad323bfbfe
Author: Markus Metzger <markus.t.metzger@intel.com>
Date:   Fri Apr 3 16:43:38 2009 +0200

    x86, bts: use trace_clock_global() for timestamps


Below is a dumb patch that gets rid of the build error. Not sure whether
this is the right fix.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>

diff --git a/arch/x86/kernel/ds.c b/arch/x86/kernel/ds.c
index 48bfe13..62b4b55 100644
--- a/arch/x86/kernel/ds.c
+++ b/arch/x86/kernel/ds.c
@@ -1377,7 +1377,9 @@ static inline void ds_take_timestamp(struct ds_context *context,
 
 	memset(&ts, 0, sizeof(ts));
 	ts.qualifier		= qualifier;
+#ifdef CONFIG_TRACING
 	ts.variant.event.clock	= trace_clock_global();
+#endif
 	ts.variant.event.pid	= task->pid;
 
 	bts_write(tracer, &ts);