Re-add #include's for header files of the C file itself

This partially reverts commit b130b35f4, "Update #include's: remove
unused and add missing ones", but fixes the following compiler and
analyzer warnings of Apple Xcode 5:

 "Semantic issue: No previous prototype for function 'yyy'"
Xcode: remove imp.h and exp.h from project file

These files have been removed from ngIRCd in commit 259c314d,
"Remove imp.h and exp.h header files".