Implement PSA ITS over files

Implement the PSA ITS API over stdio files.
diff --git a/library/Makefile b/library/Makefile
index 8533eaa..1fd159d 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -87,6 +87,7 @@
 		psa_crypto_storage.o				\
 		psa_crypto_storage_file.o			\
 		psa_crypto_storage_its.o			\
+		psa_its_file.o					\
 		ripemd160.o	rsa_internal.o	rsa.o  		\
 		sha1.o		sha256.o	sha512.o	\
 		threading.o	timing.o	version.o	\