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/bitcoin-chainstate.cpp

@@ -26,6 +26,7 @@
 #include <script/sigcache.h>
 #include <util/chaintype.h>
 #include <util/fs.h>
+#include <util/signalinterrupt.h>
 #include <util/task_runner.h>
 #include <validation.h>
 #include <validationinterface.h>