Fix GCC not compiling .c files, but .cpp

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I think I fixed it

Problem: GCC won't compile the kernel unless its code is placed in a .cpp file

From OSDev Wiki: "Writing a kernel in C++ is easy. Note that not all features from the language is available. For instance, exception support requires special runtime support and so does memory allocation. To write a kernel in C++, simply adopt code above: Add an extern "C" declaration to the main method."

Solution: Remove extern "C" from void main in kernel.cpp, and rename the file to kernel.c