oprofile: select RING_BUFFER

Impact: build fix

OProfile now depends on the ring buffer infrastructure:

 arch/x86/oprofile/built-in.o: In function `oprofile_add_ibs_sample':
 : undefined reference to `ring_buffer_unlock_commit'

Select TRACING and RING_BUFFER when oprofile is enabled.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/Kconfig

@@ -6,6 +6,8 @@ config OPROFILE
 	tristate "OProfile system profiling (EXPERIMENTAL)"
 	depends on PROFILING
 	depends on HAVE_OPROFILE
+	select TRACING
+	select RING_BUFFER
 	help
 	  OProfile is a profiling system capable of profiling the
 	  whole system, include the kernel, kernel modules, libraries,