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 <[e-mail]>
Signed-off-by: Valdis Kletnieks <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Cc: Tony Luck <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
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