KDE development package in Mandriva is libkdecore4-devel. Issue "urpmi libkdecore4-devel" and it should get installed. If urpmi complains it can't find it, try reconfiguring your rpm sources (http://easyurpmi.zarb.org/ is a good place to help you do this).
BTW. KDE headers will end up in /usr/include (I have 394 files starting with "k" and ending with ".h" there -- they all look like kde header files (version 3.4.2)).
BTW2. There is no file "ktmainwindow.h" in KDE headers, only "kmainwindow.h".
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