Once in a while a phantom error comes up that shouldn't be there, especially in a library that has been working, and you are left scratching your head. Here is a list of some last resort things to try.
Move the include file up and down
Check the end of the file before the previous include file where the error is (might be missing a semicolon at the end of the class)
Build clean
Run qmake
Delete Makefile
Delete build folders
Turn off shadow building
Close and reopen Qt Creator
Close Qt Creator, delete the .pro.user file and reopen the .pro file
Log off, and log back on
Restart the computer
Try building on a different computer
Tell a colleague, and show them the steps you did
Work on another project for a few minutes and come back to later