arm64 vdso warning: fix non-void function not returning

arch_setup_vdso() needs to return something even on panic to please gcc.
In theory, flagging panic() with __attribute__((noreturn)) should work
just the same and is a much better solution but for some reason on older
gcc versions setting the flag leads to the weak memset() symbol not
being found !?

Change-Id: Ifed100df5440ca24bb495817db9afc79f0ba6751

arch/arm64/kernel/vdso.c

@@ -87,6 +87,7 @@ int arch_setup_vdso(void)
 	}
 
 	panic("Only support host mapping vDSO");
+	return -1;
 }
 
 static int get_free_area(struct process_vm *vm, size_t len, intptr_t hint,