Fixed compiler warning in mbed_retarget.cpp
Fixed compiler warning in mbed_retarget.cpp

Fixed compiler warning about openmode_to_posix() being declared but never referenced.

j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os