[kernel] Remove placement-new declaration from memory.h.cog

Finishing the trend of using `#include <new>` to define new, get rid of
the last bits of custom-declared operator new.

src/kernel/device_manager.cpp

@@ -1,3 +1,4 @@
+#include <new>
 #include <stddef.h>
 #include <stdint.h>
 #include <util/misc.h> // for checksum