CONTRACTS: fix include error in api.h

Author: Remi Delmas

src/libcprover-cpp/api.cpp

@@ -13,6 +13,8 @@
 #include <ansi-c/ansi_c_language.h>
 #include <langapi/mode.h>
 
+#include "options.h"
+
 #include <memory>
 #include <string>
 #include <vector>

src/libcprover-cpp/api.h

@@ -10,10 +10,9 @@
 // Forward declaration of API dependencies
 class goto_modelt;
 class message_handlert;
+class api_optionst;
 class optionst;
 
-#include "options.h"
-
 // An object in the pattern of Session Facade - owning all of the memory
 // the API is using and being responsible for the management of that.
 struct api_sessiont