Fix tests build with MBEDTLS_PSA_CRYPTO_BUILTIN_KEYS enabled

In that case mbedtls_psa_platform_get_builtin_key is already declared.

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/src/drivers/platform_builtin_keys.c b/tests/src/drivers/platform_builtin_keys.c
index 01fc050..4561b6f 100644
--- a/tests/src/drivers/platform_builtin_keys.c
+++ b/tests/src/drivers/platform_builtin_keys.c
@@ -10,6 +10,8 @@
 
 #include <test/helpers.h>
 
+#if defined(MBEDTLS_PSA_CRYPTO_BUILTIN_KEYS)
+
 #include <psa/crypto.h>
 #include <psa/crypto_extra.h>
 
@@ -76,3 +78,5 @@
 
     return PSA_ERROR_DOES_NOT_EXIST;
 }
+
+#endif /* MBEDTLS_PSA_CRYPTO_BUILTIN_KEYS */