Qt Frontend: Add Threads::Threads import in CMakeLists.txt.

This had been forgotten in df81fa11fc8972a5775a57ccde1e0ef8d7fbfe64.

Fixes #1711.

src/citra_qt/CMakeLists.txt

@@ -92,7 +92,7 @@ else()
 endif()
 target_link_libraries(citra-qt core video_core audio_core common qhexedit)
 target_link_libraries(citra-qt ${OPENGL_gl_LIBRARY} ${CITRA_QT_LIBS})
-target_link_libraries(citra-qt ${PLATFORM_LIBRARIES})
+target_link_libraries(citra-qt ${PLATFORM_LIBRARIES} Threads::Threads)
 
 if(${CMAKE_SYSTEM_NAME} MATCHES "Linux|FreeBSD|OpenBSD|NetBSD")
     install(TARGETS citra-qt RUNTIME DESTINATION "${CMAKE_INSTALL_PREFIX}/bin")