winstorecompat: provide GetUserName

It's needed by:
- getlogin() in mingwex

GetEnvironmentVariableA("USERNAME") gives the same information (tested on
win10 in cmd.exe and PowerShell).

No implementation provided when building winstorecompat for win8 as
GetEnvironmentVariableA is not allowed and GetUserNameEx requires -lsecur32.

Signed-off-by: Martin Storsjö <martin@martin.st>