include/arm64: Add essentials header

Commit b85c0df ("plat/kvm/arm: Add alignament attribute to spinlock struct")
introduces the `__align()` macro in the definition of the
spinlock without including `essentials.h`.

Signed-off-by: Marc Rittinghaus <marc.rittinghaus@kit.edu>
Reviewed-by: Răzvan Vîrtan <virtanrazvan@gmail.com>
Approved-by: Michalis Pappas <mpappas@fastmail.fm>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #477

arch/arm/arm64/include/uk/asm/spinlock.h

@@ -34,6 +34,7 @@
 #error Do not include this header directly
 #endif
 
+#include <uk/essentials.h>
 #include <uk/arch/atomic.h>
 
 struct __align(8) __spinlock {