package/jpeg-turbo: force fPIC for shared libraries

When BR2_SSP_ALL is set, there is a link issue due to missing -fPIC in CFLAGS.
Set CMAKE_POSITION_INDEPENDENT_CODE=ON to add it.

This is a similar fix as for gtest package [1]

[1] https://git.buildroot.net/buildroot/commit/?id=2026621f3c60167aa8ba48e658be1b214d1347d7

Fixes:
http://autobuild.buildroot.net/results/e1f/e1f164cee16b037c0232fdda40fc16caf8f0c0af



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Murat Demirten <mdemirten@yh.com.tr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>