package/usbguard: needs std::future

package/usbguard: needs std::future

usbguard needs std::future to avoid the following build failure raised
since the addition of the package in commit
fbff7d7289cc95db991184f890f4ca1fcf8a101e:

In file included from src/Library/IPCClientPrivate.hpp:37:0,
                 from src/Library/IPCClientPrivate.cpp:24:
/home/buildroot/autobuild/run/instance-0/output-1/host/opt/ext-toolchain/arm-none-linux-gnueabi/include/c++/4.8.3/future:122:11: error: declaration of 'class std::promise<std::unique_ptr<google::protobuf::Message> >'
     class promise;
           ^
src/Library/IPCClientPrivate.cpp: In member function 'usbguard::IPC::MessagePointer usbguard::IPCClientPrivate::qbIPCSendRecvMessage(usbguard::IPC::MessageType&)':
src/Library/IPCClientPrivate.cpp:244:26: error: invalid use of incomplete type 'class std::promise<std::unique_ptr<google::protobuf::Message> >'
     auto future = promise.get_future();
                          ^
Fixes:
 - http://autobuild.buildroot.org/results/1c4e0dab5917e5559a97f7cf7a99e30a2b561fc9

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>