Makefile: Fix adding intel/common

Directory intel/common must be conditionally added in the list
of source directories, as the parent directory southbridge/intel
is unconditionally added even for boards without such device.

Change-Id: I7088bc6db9f56909ffa996aa7eff76cd72e177eb
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3827
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/southbridge/intel/common/Kconfig

@@ -0,0 +1,2 @@
+config SOUTHBRIDGE_INTEL_COMMON
+	def_bool n