Remove _Thread_local variable for 2.28

We do not intend to support multithreaded testing in 2.28, so
introducing a C11 feature here is an unnecessary burden.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/tests/src/test_memory.c b/tests/src/test_memory.c
index ac9dde6..9da7f20 100644
--- a/tests/src/test_memory.c
+++ b/tests/src/test_memory.c
@@ -20,7 +20,7 @@
 
 #if defined(MBEDTLS_TEST_MEMORY_CAN_POISON)
 
-_Thread_local unsigned int mbedtls_test_memory_poisoning_count = 0;
+unsigned int mbedtls_test_memory_poisoning_count = 0;
 
 static void align_for_asan(const unsigned char **p_ptr, size_t *p_size)
 {