The file

Qt5TextToSpeech_QTextToSpeechPluginIos.cmake

is not included by the parent cmake file 

Qt5TextToSpeechConfig.cmake

because of the pattern of the file Qt5TextToSpeech_QTextToSpeechPluginIos.cmake does not match the pattern in Qt5TextToSpeechConfig.cmake that is (line ~156)

file(GLOB pluginTargets "${CMAKE_CURRENT_LIST_DIR}/Qt5TextToSpeech_*Plugin.cmake")

Changing this to (note the end star)

file(GLOB pluginTargets "${CMAKE_CURRENT_LIST_DIR}/Qt5TextToSpeech_*Plugin*.cmake")

or renaming Qt5TextToSpeech_QTextToSpeechPluginIos.cmake to a more consistent naming scheme such as Qt5TextToSpeech_QTextToSpeechIosPlugin.cmake
solves the issue.