retarget: Added workaround for conflicting definitions of error_t
retarget: Added workaround for conflicting definitions of error_t

The errno.h header file defines the type error_t, unfortunately this
is a common type name that may be defined in user code. For at least GCC
we can work around this by telling errno that the error_t is already
defined.

mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,betzw/mbed-os,andcor02/mbed-os,mbedmicro/mbed,betzw/mbed-os,betzw/mbed-os,c1728p9/mbed-os,betzw/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,andcor02/mbed-os