Use the correct freetype include directory to avoid a build failure.

Gentoo bug #467998.

--- CMakeLists.txt
+++ CMakeLists.txt
@@ -11,7 +11,7 @@
 
 configure_file(config-kpovmodeler.h.cmake ${CMAKE_CURRENT_BINARY_DIR}/config-kpovmodeler.h )
 
-include_directories(${FREETYPE_INCLUDE_DIR})
+include_directories(${FREETYPE_INCLUDE_DIRS})
 add_definitions (-DQT3_SUPPORT -DQT3_SUPPORT_WARNINGS)
 
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