jpeg-turbo: fix install with a static only lib

jpeg-turbo: fix install with a static only lib

Define CMAKE_INSTALL_RPATH only if ENABLE_SHARED is set otherwise the
following error is raised when installing binaries:

CMake Error at cmake_install.cmake:73 (file):
  file RPATH_CHANGE could not write new RPATH:

    /usr/lib

  to the file:

    /home/fabrice/buildroot/output/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/bin/rdjpgcom

  No valid ELF RPATH or RUNPATH entry exists in the file;

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>