signals: Temporarily add boost headers to bitcoind and bitcoin-node builds

The current code forward-declares boost::signals2, which avoids the need for
these includes.

An upcoming commit will (temporarily) include boost headers directly instead.

A follow-up commit will then replace boost with an internal signals
implementation, which will allow this commit to be reverted.

src/CMakeLists.txt

@@ -307,6 +307,7 @@ if(BUILD_DAEMON)
   target_link_libraries(bitcoind
     core_interface
     bitcoin_node
+    Boost::headers
     $<TARGET_NAME_IF_EXISTS:bitcoin_wallet>
   )
   install_binary_component(bitcoind HAS_MANPAGE)
@@ -320,6 +321,7 @@ if(ENABLE_IPC AND BUILD_DAEMON)
     core_interface
     bitcoin_node
     bitcoin_ipc
+    Boost::headers
     $<TARGET_NAME_IF_EXISTS:bitcoin_wallet>
   )
   install_binary_component(bitcoin-node INTERNAL)