fix -Wold-style cast on RHEL 7

This is an interesting bug. Other than the simple 'old-style cast' fix.

When compiling on RHEL 7, with the cmake option: `-DUSE_CCACHE=yes` the build was successful and there was no errors.

However with the plain `cmake ..` we would get build errors such as:
```
/home/otdev/opentxs/src/core/crypto/OTSymmetricKey.cpp:1010:236: error: use of old-style cast [-Werror=old-style-cast]
```

Yet, if one did a successful build with the `-DUSE_CCACHE=yes` then deleted the build directory and rerun with `cmake ..`  the build would be still successful.

One needs to do the command:
```
ccache -C
```

for this bug to re-appears.