MdeModulePkg: Fix constructor invocation ordering

MdeModulePkg: Fix constructor invocation ordering

The BaseSerialPortLib16550 library does not implement
a constructor. This prevents the correct constructor
invocation order for dependent libraries.
e.g. A PlatformHookLib (for the Serial Port) may have
a dependency on retrieving data from a Hob. A Hob
library implementation may configure its initial state
in the HobLib constructor. Since BaseSerialPortLib16550
does not implement a constructor, the Basetools do not
resolve the correct order for constructor invocation.

To fix this, add an empty constructor to the serial port
library BaseSerialPortLib16550.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Ard Biesheuvel <Ard.Biesheuvel@arm.com>
Acked-by: Hao A Wu <hao.a.wu@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>