Make include guards unique (#539297)

Include guards need to be unique within GParted code and all included
library header files.
    http://en.wikipedia.org/wiki/Include_guard#Difficulties

Use this model for all include guards:
    #ifndef GPARTED_FILE_NAME_H
    #define GPARTED_FILE_NAME_H
    ...
    #endif /* GPARTED_FILE_NAME_H */

Closes Bug #539297 - Make include guards unique