package/libraw: add Libs.private to libraw.pc

Add Libs.private to libraw.pc to fix the following static build failure
with imagemagick which is raised since commit
2f47cfad:

/home/giuliobenetti/autobuild/run/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/9.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/giuliobenetti/autobuild/run/instance-1/output-1/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libjasper.a(jpg_enc.c.o): in function `jpg_encode':
jpg_enc.c:(.text+0x1f4): undefined reference to `jpeg_stdio_dest'

Fixes:
 - http://autobuild.buildroot.org/results/88e43a1ea2059a684e50b0f5f2af407e8c6df2e1



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>