include: nuttx: Introduce spinlock_t for #ifndef CONFIG_SPINLOCK

Summary:
- This commit introduces spinlock_t for #ifndef CONFIG_SPINLOCK
  which is useful for the non-SMP case because it does not consume
  memory

Impact:
- None:

Testing:
- N/A

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

include/nuttx/spinlock.h

@@ -30,7 +30,11 @@
 #include <sys/types.h>
 #include <stdint.h>
 
-#ifdef CONFIG_SPINLOCK
+#ifndef CONFIG_SPINLOCK
+typedef struct
+{
+} spinlock_t;
+#else
 
 /* The architecture specific spinlock.h header file must also provide the
  * following: