fix "make install" failure

before:

CMake Error at librepo/python/python2/cmake_install.cmake:62 (FILE):
  file INSTALL cannot find
  "/home/zpavlas/GIT/librepo/build/librepo/python/python2/librepo/_librepomodule.so".
Call Stack (most recent call first):
  librepo/python/cmake_install.cmake:37 (INCLUDE)
  librepo/cmake_install.cmake:107 (INCLUDE)
  cmake_install.cmake:37 (INCLUDE)

after:

-- Installing: /usr/lib/python2.7/site-packages/librepo/_librepomodule.so
-- Removed runtime path from "/usr/lib/python2.7/site-packages/librepo/_librepomodule.so"