Remove extern "C" from fake declarations.

Assuming your C interfaces are appropriately wrapped for C++, it is
unnecessary to force the fakes to be declared extern "C", and doing so
causes any functions declared with C++ linkage to be impossible to fake
due to the conflicting linkage declarations.

examples/driver_testing/driver.test.fff.cpp

@@ -1,6 +1,7 @@
 extern "C"{
     #include "driver.h"
     #include "registers.h"
+    #include "hardware_abstraction.h"
 }
 #include "../../fff.h"
 #include <gtest/gtest.h>