cmake: Require Qt if UI is enabled

In CMakeLists.txt, FIND_MODE is not defined here. This was moved from
UI/CMakeLists.txt in commit cb4d2ff7. In
UI/CMakeLists.txt, FIND_MODE is REQUIRED if DISABLE_UI is false or
undefined and ENABLE_UI is true. Since the same booleans are required
for the if-else branch in CMakeLists.txt where we try to find Qt, we can
set find_package to REQUIRED here as well.

CMakeLists.txt

@@ -254,7 +254,7 @@ if(NOT INSTALLER_RUN)
 			list(APPEND CMAKE_PREFIX_PATH "$ENV{QTDIR}")
 		endif()
 
-		find_package(Qt5Widgets ${FIND_MODE})
+		find_package(Qt5Widgets REQUIRED)
 	endif()
 
 	add_subdirectory(deps)