CMakeLists: Explicitly link with pthread

:Release Notes:
Building filecache can fail because of missing pthread_atfork symbol.
Make sure that pthread is explicitly declared here.

:Detailed Notes:
4.8.2/ld: filecache: hidden symbol `pthread_atfork' in
/usr/lib/libpthread_nonshared.a(pthread_atfork.oS) is referenced by DSO
4.8.2/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status

For details why this happens see:
http://ryanarn.blogspot.cz/2011/07/curious-case-of-pthreadatfork-on.html

:Testing Performed:
Only build tested.

:QA Notes:
No change to image.

:Issues Addressed:
[ANG-329] Ongoing metadata improvement/cleanup work

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I7d3deba34114867bf5cd2a2226b7644000ce380c
Reviewed-on: https://g2g.palm.com/4831
Reviewed-by: DCO Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>