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>

fs/fs.c

@@ -288,6 +288,7 @@ static struct fstype_info fstypes[] = {
 	},
 #endif
 #endif
+#ifndef CONFIG_SPL_BUILD
 #ifdef CONFIG_FS_BTRFS
 	{
 		.fstype = FS_TYPE_BTRFS,
@@ -307,6 +308,7 @@ static struct fstype_info fstypes[] = {
 		.ln = fs_ln_unsupported,
 	},
 #endif
+#endif
 #if IS_ENABLED(CONFIG_FS_SQUASHFS)
 	{
 		.fstype = FS_TYPE_SQUASHFS,