Fix install include directory

Specifing the path twice would cause CMake to think that the include
files were being installed to "{PREFIX}/DESTINATION", which would cause
package inclusion to fail.

src/CMakeLists.txt

@@ -105,7 +105,6 @@ install(TARGETS argtable3
   RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
   LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
   ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
-  INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
 )
 
 install(FILES "${PROJECT_SOURCE_DIR}/src/argtable3.h"