Check _DEBUG is defined before using it

to avoid the preprocess warning

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: I4ae1eb8533563d377ec8614f0c9428c8734e1f2c

tlsf.c

@@ -1045,8 +1045,8 @@ void tlsf_remove_pool(tlsf_t tlsf, pool_t pool)
 ** TLSF main interface.
 */
 
-#if _DEBUG
-int test_ffs_fls()
+#if defined(_DEBUG) && _DEBUG
+static int test_ffs_fls()
 {
 	/* Verify ffs/fls work properly. */
 	int rv = 0;
@@ -1075,7 +1075,7 @@ int test_ffs_fls()
 
 tlsf_t tlsf_create(void* mem)
 {
-#if _DEBUG
+#if defined(_DEBUG) && _DEBUG
 	if (test_ffs_fls())
 	{
 		return 0;