Fix linking error for mbedtls_test_hook_error_add

This commit fixes a linking error with psa_sim_crypto_server.c where
mbedtls_test_hook_error_add was undefined.

Signed-off-by: Harry Ramsey <harry.ramsey@arm.com>
diff --git a/tests/psa-client-server/psasim/src/psa_sim_crypto_server.c b/tests/psa-client-server/psasim/src/psa_sim_crypto_server.c
index b2ed070..2daacd8 100644
--- a/tests/psa-client-server/psasim/src/psa_sim_crypto_server.c
+++ b/tests/psa-client-server/psasim/src/psa_sim_crypto_server.c
@@ -21,6 +21,10 @@
 #error "Error: MBEDTLS_PSA_CRYPTO_C must be enabled on server build"
 #endif
 
+#if defined(MBEDTLS_TEST_HOOKS)                                                 
+void (*mbedtls_test_hook_error_add)(int, int, const char *, int);               
+#endif
+
 // Returns 1 for success, 0 for failure
 int psa_crypto_init_wrapper(
     uint8_t *in_params, size_t in_params_len,