RAS: Build debugfs.o only when enabled in Kconfig

In addition, the 0day bot reported this build error:

  >> drivers/ras/debugfs.c:10:5: error: redefinition of 'ras_userspace_consumers'
      int ras_userspace_consumers(void)
          ^~~~~~~~~~~~~~~~~~~~~~~
     In file included from drivers/ras/debugfs.c:3:0:
     include/linux/ras.h:14:19: note: previous definition of 'ras_userspace_consumers' was here
      static inline int ras_userspace_consumers(void) { return 0; }
                      ^~~~~~~~~~~~~~~~~~~~~~~

for a riscv-specific .config where CONFIG_DEBUG_FS is not set. Fix all
that by making debugfs.o depend on that define.

 [ bp: Rewrite commit message. ]
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-edac@vger.kernel.org
Cc: x86@kernel.org
Link: http://lkml.kernel.org/r/7053.1565218556@turing-police

drivers/ras/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-obj-$(CONFIG_RAS)	+= ras.o debugfs.o
+obj-$(CONFIG_RAS)	+= ras.o
+obj-$(CONFIG_DEBUG_FS)	+= debugfs.o
 obj-$(CONFIG_RAS_CEC)	+= cec.o