kernel: Remove key module from kernel library

The key module's functionality is not used by the kernel library, but
currently kernel users are still required to initialize the key module's
`secp256k1_context_sign` global as part of the `kernel::Context` through
`ECC_Start`.

src/node/kernel_notifications.cpp

@@ -14,6 +14,7 @@
 #include <node/abort.h>
 #include <node/interface_ui.h>
 #include <util/check.h>
+#include <util/signalinterrupt.h>
 #include <util/strencodings.h>
 #include <util/string.h>
 #include <util/translation.h>