Fix imported CAPI function declarations

[smlu]

When including testing framework, some eosio C++ headers conflict with C headers and you get compiler error: conflicting types for .... The issue is wrong declaration of some CAPI functions in C++ headers. Namely, declaration of send_deferred and cancel_deferred CAPI functions. This PR fixes those incorrect declaration.

PR fix partially #811.

Note: The issue still persists for functions preactivate_feature and is_feature_activated due to struct capi_checksum256 declared in eosio/system.hpp#L17, although scoped in extern "C", still defines different type than struct defined in capi/types.h#L51