sysfs: Unbreak the build around sysfs_bin_attr_simple_read()

Günter reports build breakage for m68k "m5208evb_defconfig" plus
CONFIG_BLK_DEV_INITRD=y caused by commit 66bc1a17 ("treewide:
Use sysfs_bin_attr_simple_read() helper").

The defconfig disables CONFIG_SYSFS, so sysfs_bin_attr_simple_read()
is not compiled into the kernel.  But init/initramfs.c references
that function in the initializer of a struct bin_attribute.

Add an empty static inline to avoid the build breakage.

Fixes: 66bc1a17

 ("treewide: Use sysfs_bin_attr_simple_read() helper")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/r/e12b0027-b199-4de7-b83d-668171447ccc@roeck-us.net
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/05f4290439a58730738a15b0c99cd8576c4aa0d9.1716461752.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>