include: rb: doc: Move code to make Doxygen happy

Moved alloca.h trick further down the file to workaround Doxygen
getting confused with #ifdef/#ifndef.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

include/zephyr/sys/rb.h

@@ -4,18 +4,6 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-/* Our SDK/toolchains integration seems to be inconsistent about
- * whether they expose alloca.h or not.  On gcc it's a moot point as
- * it's always builtin.
- */
-#ifdef __GNUC__
-#ifndef alloca
-#define alloca __builtin_alloca
-#endif
-#else
-#include <alloca.h>
-#endif
-
 /**
  * @file
  * @defgroup rbtree_apis Balanced Red/Black Tree
@@ -52,6 +40,18 @@
 #include <stdbool.h>
 #include <stdint.h>
 
+/* Our SDK/toolchains integration seems to be inconsistent about
+ * whether they expose alloca.h or not.  On gcc it's a moot point as
+ * it's always builtin.
+ */
+#ifdef __GNUC__
+#ifndef alloca
+#define alloca __builtin_alloca
+#endif
+#else
+#include <alloca.h>
+#endif
+
 /**
  * @brief Balanced red/black tree node structure
  */