You just have to add ‘#include ’ at the top of the stm32f4xx_it.c file...
The only thing is that you must save the file before adding references to uint32_t in the file, or you will get this error as System Workbench will only re-index the file when saved (to save computing power); the error will disapear when saving the file. Note that this error is not a compiler error, but an error reported by System Workbench for STM32 “code-assist” feature.
As a rule of thumb it’s thus always better to save after a modification in the include directives, otherwise System Workbench will not be able to “see” the content of the added files.
Note that on System Workbench, you can save your files as often as you want: you will always be able to roll-back modifications using the “Local history” feature: by “file >> Compare With >> Local history...” you can see all intermediate versions of your file and see what changed from this version, rollback incorrect changes, etc...
If this behavior is disturbing you too much, and you have a powerful enough CPU, you can click the “Index source and header files opened in editor” check-box in “Window >> Preferences >> C/C++ >> Indexer”; Instead of changing this globally you can also change it for a single project in “project >> Properties >> C/C++ General >> Indexer” (after clicking “Enable project specific settings” of course).