Declare the new slot management functions in crypto.h

No changes to existing functions.
diff --git a/include/psa/crypto_platform.h b/include/psa/crypto_platform.h
index 9af320d..c203966 100644
--- a/include/psa/crypto_platform.h
+++ b/include/psa/crypto_platform.h
@@ -49,4 +49,7 @@
 /* Integral type representing a key slot number. */
 typedef uint16_t psa_key_slot_t;
 
+/* Integral type representing a key handle. */
+typedef uint16_t psa_key_handle_t;
+
 #endif /* PSA_CRYPTO_PLATFORM_H */