cmake: fixes for cmake 3.8

AUTOUIC seems to became stricter.

src/gui/CMakeLists.txt

@@ -135,7 +135,7 @@ shutdownconfirmdlg.ui
 
 qbt_target_sources(about.qrc)
 
-add_library(qbt_gui STATIC ${QBT_GUI_HEADERS} ${QBT_GUI_SOURCES})
+add_library(qbt_gui STATIC ${QBT_GUI_HEADERS} ${QBT_GUI_SOURCES} ${QBT_GUI_FORMS})
 target_link_libraries(qbt_gui qbt_lineedit qbt_powermanagement qbt_rss qbt_properties qbt_searchengine
     ${QBT_GUI_OPTIONAL_LINK_LIBRARIES} qbt_base
     QtSingleApplication::QtSingleApplication

src/gui/properties/CMakeLists.txt

@@ -42,6 +42,6 @@ speedwidget.cpp
 speedplotview.cpp
 )
 
-add_library(qbt_properties STATIC ${QBT_PROPERTIES_HEADERS} ${QBT_PROPERTIES_SOURCES})
+add_library(qbt_properties STATIC ${QBT_PROPERTIES_HEADERS} ${QBT_PROPERTIES_SOURCES} ${QBT_PROPERTIES_FORMS})
 target_link_libraries(qbt_properties qbt_base)
 target_link_libraries(qbt_properties Qt5::Widgets Qt5::Concurrent)