Added motor adjustment code

This code will adjust the micromouse as we move forward. The
motor_adjustment variable is set by the detection code and read
by the motor code to determind if we need any adjustments to center
the mouse in the maze.
Fixed global variable

The extern keyword only delcares the variable but doesn't allocate
any memory address to it. We need to give it a place in memory
so that it can be stored. Do it in the motor.c file since it is
delcare in the motor.h file.
Fixed missing brace