Cellular: Fix ThisThread::sleep_until link error

ATHandler::cmd_start (ATHandler.c) calls ThisThread::sleep_until, which
has parameter abs_time, whose type is Clock::time_point. Clock::time_point
type has different definitions dependent on MBED_CONF_RTOS_PRESENT defined
or not (rtos/Kernel.h). For cellular application whose executable cmake
target always links mbed-os rather than mbed-baremetal, mbed-cellular must
also link mbed-rtos-flags to be consistent with executable, so that both
have MBED_CONF_RTOS_PRESENT defined.

connectivity/cellular/CMakeLists.txt

@@ -39,5 +39,6 @@ target_link_libraries(mbed-cellular
     PUBLIC
         mbed-netsocket-api
         mbed-core-flags
+        mbed-rtos-flags
         mbed-randlib
 )