psa: Fix builds without MBEDTLS_PLATFORM_C

When `MBEDTLS_PLATFORM_C` is not enabled, our PSA Crypto implementation
depends on the standard C library for functions like snprintf() and
exit(). However, our implementation was not including the proper header
files nor redefining all `mbedtls_*` symbols properly to ensure
successful builds without MBEDTLS_PLATFORM_C. Add the necessary header
files and macro definitions to our PSA Crypto implementation.
diff --git a/library/psa_crypto_storage.c b/library/psa_crypto_storage.c
index 687269b..b4e4076 100644
--- a/library/psa_crypto_storage.c
+++ b/library/psa_crypto_storage.c
@@ -38,6 +38,7 @@
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
 #else
+#include <stdlib.h>
 #define mbedtls_calloc   calloc
 #define mbedtls_free     free
 #endif