Windows/DynamicLibrary.inc: Remove 'extern "C"' in ELM_Callback.

'extern "C" static' is not accepted by g++-4.7. Rather to tweak, I just removed 'extern "C"', since it doesn't affect the ABI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218290 91177308-0d34-0410-b5e6-96231b3b80d8