Add conditional error.h include to test helpers function.

`mbedtls_test_hook_error_add` is referenced inside main_test.function.
Including the `error.h` is necessary to build suites which define both
MBEDTLS_TEST_HOOKS and MBEDTLS_ERROR_C, such as:
build_psa_accel_alg_ecdh

Signed-off-by: Mateusz Starzyk <mateusz.starzyk@mobica.com>
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index ca03532..ae1bc88 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -11,6 +11,9 @@
 
 #include <stdlib.h>
 
+#if defined (MBEDTLS_ERROR_C)
+#include "mbedtls/error.h"
+#endif
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
 #else