Fix exporting of Targets.cmake for include dir

[drTr0jan]

Description

Building main example from Readme via CMake (CMakeLists.txt) using doctestTargets.cmake causes an error.

boris@boris:~/doctest% cmake --build .
[ 50%] Building CXX object CMakeFiles/main.dir/main.cpp.o
/home/boris/doctest/main.cpp:2:10: fatal error: 'doctest.h' file not found
#include "doctest.h"
         ^~~~~~~~~~~
1 error generated.
gmake[2]: *** [CMakeFiles/main.dir/build.make:76: CMakeFiles/main.dir/main.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/main.dir/all] Error 2
gmake: *** [Makefile:91: all] Error 2

Proposed patch specifies doctest directory explicit for INCLUDES DESTINATION.

Main example builds with the patched doctestTargets.cmake successful.

GitHub Issues

For #175.