Make legacy_or_psa.h public.

As a public header, it should no longer include common.h, just use
build_info.h which is what we actually need anyway.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index 8777833..b6a8add 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -38,7 +38,7 @@
 #include "mbedtls/platform_util.h"
 #include "mbedtls/constant_time.h"
 
-#include "legacy_or_psa.h"
+#include "mbedtls/legacy_or_psa.h"
 
 #include <string.h>