package/uhd: fix build with gcc 11

package/uhd: fix build with gcc 11

Fix the following build failure with gcc 11:

In file included from /tmp/instance-4/output-1/build/uhd-3.15.0.0/host/lib/usrp/dboard/e3xx/e3xx_ad9361_iface.hpp:14,
                 from /tmp/instance-4/output-1/build/uhd-3.15.0.0/host/lib/usrp/dboard/e3xx/e3xx_ad9361_iface.cpp:7:
/tmp/instance-4/output-1/build/uhd-3.15.0.0/host/lib/include/uhdlib/utils/rpc.hpp: In function 'bool {anonymous}::await_rpc_connected_state(std::shared_ptr<rpc::client>, std::chrono::milliseconds)':
/tmp/instance-4/output-1/build/uhd-3.15.0.0/host/lib/include/uhdlib/utils/rpc.hpp:28:27: error: 'sleep_for' is not a member of 'std::this_thread'
   28 |         std::this_thread::sleep_for(std::chrono::milliseconds(1));
      |                           ^~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/1591ff7fabb09e4eb79f3aa293fbb228c255c45b

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>