Added extern "C" declaration to MockGetPlatformSpecificTimeInMillis().

The cl2000 compiler is quite fussy when it comes to function pointers. If a method expects a "C" function pointer, you cannot pass it a "C++" function pointer, and vice versa.

Since you cannot use 'extern "C"' inline, it can be necessary to declare suitable C function pointer types, in order to cast non-C function pointers.