Fix a multiple include. The += adds a new item to the list, whereas *= only
adds it if it is not already present.
Replace all INCLUDEPATH += $$PWD by INCLUDEPATH *= $$PWD
Added another INCLUDEPATH that was being pulled in multiple times in the
plugins.
Changed all remaining INCLUDEPATH += to INCLUDEPATH *=
Merge pull request #618 from TauLabs/multiple-include-fix

Multiple include fix