openjpeg: fix static linking of liblcms2

Fixes [1]:

  .../host/usr/bfin-buildroot-uclinux-uclibc/sysroot/usr/lib/liblcms2.a(cmsplugin.o): In function `_cmsDeleteContext':
  cmsplugin.c:(.text+0x1c2): undefined reference to `_pthread_mutex_lock'
  cmsplugin.c:(.text+0x1f6): undefined reference to `_pthread_mutex_unlock'

http://autobuild.buildroot.net/results/5ce/5cee20afd8bef5268832cddcb3a5270746be7a57



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>