fs: Allow to compile FS_BTRFS when SPL is enabled

Currently there is no btrfs support in SPL. But macro CONFIG_FS_BTRFS is
defined also when building SPL. When both FS_BTRFS and SPL are enabled
then build process throw compile error.

Fix check for btrfs code in fstypes[] to allow compiling FS_BTRFS only in
proper U-Boot.

Signed-off-by: Pali Rohár <pali@kernel.org>