HAL: Use extern "C" in implementation files.

This turns accidental parameter mismatches between header and implementation
into compiler errors.

Change-Id: Ic26fabb82b2fd5f79407a11435cdbd35348af15f

hal/lib/Athena/Accelerometer.cpp

@@ -171,6 +171,8 @@ static double unpackAxis(int16_t raw) {
 	}
 }
 
+extern "C" {
+
 /**
  * Set the accelerometer to active or standby mode.  It must be in standby
  * mode to change any configuration.
@@ -232,3 +234,5 @@ double getAccelerometerZ() {
 	int raw = (readRegister(kReg_OutZMSB) << 4) | (readRegister(kReg_OutZLSB) >> 4);
 	return unpackAxis(raw);
 }
+
+}  // extern "C"