Fix build in out-of-source builds

Summary:
Code seemed right, but for some reason the folder path was empty:
[ 5%] Building CXX object CMakeFiles/kdevclangtidy.dir/src/plugin.cpp.o
/usr/bin/c++ -DKCOREADDONS_LIB -DQT_CORE_LIB -DQT_DBUS_LIB
-DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_NO_DEBUG -DQT_WIDGETS_LIB
-DQT_XML_LIB -D_GNU_SOURCE -D_LARGEFILE64_SOURCE -Dkdevclangtidy_EXPORTS
-I/home/tcanabrava/Projects/kdesrc/build/kdev-clang-tidy
-I/home/tcanabrava/Projects/kdesrc/kdev-clang-tidy -I/src

the -I/src should have been:

-I/home/tcanabrava/Projects/kdesrc/kdev-clang-tidy/src

my setup is:

Projects/sources/blah
Projects/builds/blah

Maybe the current way worked with inner build folders.

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3730