Fix compile error with DirectSerial in serial-less build

[ccli8]

Description

This PR tries to fix compile error with DirectSerial in serial-less build. My mbed_app.json has the following content for this build:

{
    "target_overrides": {
        "*": {
            "platform.stdio-baud-rate": 9600,
            "platform.stdio-convert-newlines": true
        },
        
        "NUMAKER_PFM_M2351_S": {
            "target.device_has_remove": ["SERIAL", "SERIAL_ASYNCH", "SERIAL_FC", "STDIO_MESSAGES"]
        
        }
    }
}

Related PR

#6747

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[kjbracey]

Thanks! Surprised we'd not run into that sooner.

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1898
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6797/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1545
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6797/

[mbed-ci]

Test : SUCCESS

Build number : 1715
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6797/1715