Fix for GCC6 error

[marki555]

Compiling with GCC6 is not possible due to several reported errors:

First one can be fixed by this simple change: monero-project/monero@6462a3a

Other errors can be converted into warnings by this workaround: https://github.com/dinastyoffreedom/dinastycoin/issues/6

After these changes I was able to compile under Debian Stretch with gcc v6.3.0.