perf: Fix unexported generic perf_arch_fetch_caller_regs

perf: Fix unexported generic perf_arch_fetch_caller_regs

perf_arch_fetch_caller_regs() is exported for the overriden x86
version, but not for the generic weak version.

As a general rule, weak functions should not have their symbol
exported in the same file they are defined.

So let's export it on trace_event_perf.c as it is used by trace
events only.

This fixes:

	ERROR: ".perf_arch_fetch_caller_regs" [fs/xfs/xfs.ko] undefined!
	ERROR: ".perf_arch_fetch_caller_regs" [arch/powerpc/platforms/cell/spufs/spufs.ko] undefined!

-v2: And also only build it if trace events are enabled.
-v3: Fix changelog mistake

Reported-by: Stephen Rothwell <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Xiao Guangrong <[email protected]>
Cc: Paul Mackerras <[email protected]>
LKML-Reference: <1268697902[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>