refactor: clean up external_signer.h includes

Co-Authored-By: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

src/qt/walletcontroller.cpp

@@ -11,6 +11,7 @@
 #include <qt/guiutil.h>
 #include <qt/walletmodel.h>
 
+#include <external_signer.h>
 #include <interfaces/handler.h>
 #include <interfaces/node.h>
 #include <util/string.h>