Kernel/riscv64: Make the kernel compilable using GCC

This commit adds all necessary includes, so all functions are properly
declared.
PCI.cpp is moved to PCI/Initializer.cpp, as that matches the header
path.

Kernel/Arch/riscv64/DebugOutput.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <Kernel/Arch/DebugOutput.h>
 #include <Kernel/Library/Assertions.h>
 
 namespace Kernel {