Kernel: Move SpinLock definition to header and fix Scheduler locking

This patch allows inlining of spinlocks :)

kernel/CMakeLists.txt

@@ -42,7 +42,6 @@ set(KERNEL_SOURCES
 	kernel/Input/PS2/Mouse.cpp
 	kernel/InterruptController.cpp
 	kernel/kernel.cpp
-	kernel/Lock/SpinLock.cpp
 	kernel/Memory/DMARegion.cpp
 	kernel/Memory/FileBackedRegion.cpp
 	kernel/Memory/Heap.cpp