qt: Don't require db_cxx.h when wallet disabled

Fix #3978.

src/qt/Makefile.am

@@ -276,7 +276,6 @@ BITCOIN_QT_CPP = \
   qvalidatedlineedit.cpp \
   qvaluecombobox.cpp \
   rpcconsole.cpp \
-  signverifymessagedialog.cpp \
   splashscreen.cpp \
   trafficgraphwidget.cpp \
   utilitydialog.cpp
@@ -298,6 +297,7 @@ BITCOIN_QT_CPP += \
   recentrequeststablemodel.cpp \
   sendcoinsdialog.cpp \
   sendcoinsentry.cpp \
+  signverifymessagedialog.cpp \
   transactiondesc.cpp \
   transactiondescdialog.cpp \
   transactionfilterproxy.cpp \

src/qt/bitcoin.cpp

@@ -25,7 +25,9 @@
 #include "rpcserver.h"
 #include "ui_interface.h"
 #include "util.h"
+#ifdef ENABLE_WALLET
 #include "wallet.h"
+#endif
 
 #include <stdint.h>