How serious this is depends on whether PhotoshopSDK.h contained significant
source definitions or, as I suspect, it was just a wrapper that included
several of the SDK files.
One of your most important tools is the ability to recursively search a
source tree for a particular symbol definition, then add that .h file and
compile again, etc. The Windows Visual Studio search command makes this
fairly easy to do.
If you're on the Mac, I imagine there is a similar tool. In a pinch the
grep command will get the job done. If you fire up a terminal window, the
following command will do a recursive search:
find . -exec grep -i blah {}\;
(where blah is the symbol you're looking for)