explicitly include iostream to compile with boost >= 1.65.0

When compiling with boost version >= 1.65.0, it fails with:
```
In file included from /[...]/include/socketcan_interface/asio_base.h:4:0,
                 from /[...]/include/socketcan_interface/socketcan.h:4,
                 from /[...]/src/socketcan_interface_plugin.cpp:2:
/[...]/include/socketcan_interface/socketcan.h: In member function 'virtual bool can::SocketCANInterface::enqueue(const can::Frame&)':
/[...]/include/socketcan_interface/interface.h:202:96: error: 'cout' is not a member of 'std'
 #define LOG(log) { boost::mutex::scoped_lock _cout_lock(_cout_wrapper::get_cout_mutex()); std::cout << log << std::endl; }
                                                                                                ^
/[...]/include/socketcan_interface/socketcan.h:183:13: note: in expansion of macro 'LOG'
             LOG("FAILED " << ec);
             ^
/[...]/include/socketcan_interface/interface.h:202:96: note: suggested alternative: 'count'
```

With boost version < 1.65.0, iostream is probably included by the
boost dependencies in the header files; with boost version >= 1.65.0,
it is not included anymore and hence leads to this compile error.

The issue was identified by the work on meta-ros, the OpenEmbedded
layer for ROS [1,2]. It provides a cross-compilation tool chain for
ROS packages and its current master version already uses boost
version 1.65.1.

[1] https://github.com/bmwcarit/meta-ros
[2] bulwahn/meta-ros#8

Signed-off-by: Lukas Bulwahn <[email protected]>

socketcan_interface/include/socketcan_interface/interface.h

@@ -190,6 +190,7 @@ class DriverInterface : public CommInterface, public StateInterface {
 
 } // namespace can
 
+#include <iostream>
 #include <boost/thread/mutex.hpp>
 
 struct _cout_wrapper{