btrfs-progs: kerncompat: define __DECLARE_FLEX_ARRAY if missing

The macro is not available on LTS distros, copy definition from kernel.

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

include/kerncompat.h

@@ -103,6 +103,26 @@
 #define BUILD_ASSERT(x)
 #endif
 
+#ifndef __DECLARE_FLEX_ARRAY
+/*
+ * Copied from linux.git/include/uapi/stddef.h
+ *
+ * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
+ *
+ * @TYPE: The type of each flexible array element
+ * @NAME: The name of the flexible array member
+ *
+ * In order to have a flexible array member in a union or alone in a struct, it
+ * needs to be wrapped in an anonymous struct with at least 1 named member, but
+ * that member can be empty.
+ */
+#define __DECLARE_FLEX_ARRAY(TYPE, NAME)	\
+	struct { \
+		struct { } __empty_ ## NAME; \
+		TYPE NAME[]; \
+	}
+#endif
+
 static inline void print_trace(void)
 {
 #ifndef BTRFS_DISABLE_BACKTRACE