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/rpc/mining.cpp

@@ -31,6 +31,7 @@
 #include <script/signingprovider.h>
 #include <txmempool.h>
 #include <univalue.h>
+#include <util/signalinterrupt.h>
 #include <util/strencodings.h>
 #include <util/string.h>
 #include <util/time.h>