nsAppFileLocationProvider.cpp: use of undeclared identifier 'XRE_EXECUTABLE_FILE'

Last Windows builds failed:

 9:00.60 E /var/tmp/build/firefox-3d0732cd386e/xpcom/io/nsAppFileLocationProvider.cpp:177:30: error: use of undeclared identifier 'XRE_EXECUTABLE_FILE'
 9:00.60 E   177 |   rv = directoryService->Get(XRE_EXECUTABLE_FILE, NS_GET_IID(nsIFile),
 9:00.60 E       |                              ^~~~~~~~~~~~~~~~~~~

Probably, so far we've been lucky and unified sources always pulled in the correct file, or there was an upstream refactor that removed it (though it doesn't seem the case according to git log -S).

I think we can call it a desktop or an all-platforms bug though, since that code path should be in all platforms (for the portable mode, which isn't enabled in Android).