Clean up UARTSerial.cpp includes.

[tkem]

Description

I noticed that every driver .cpp file includes at least its corresponding header file before any device feature tests such as #if DEVICE_SERIAL, with the notable exception of UARTSerial.cpp. Even if the include is not necessary for the DEVICE macros to be present, for the sake of consistency I think UARTSerial.cpp should unconditionally include UARTSerial.h.

UARTSerial.cpp also includes <errno.h>, but apparently does not use it, so I think it's safe to remove that, too.

Pull request type

[ ] Fix
[X] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Nothing's broken, so I think "Refactor" describes this best (you may want to add "Code Cleanup" as an option some day).

[0xc0170]

/morph build

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3436
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8489/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3057
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8489/

[mbed-ci]

Test : SUCCESS

Build number : 3225
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8489/3225

[cmonr]

/morph mbed2-build