package/gd: fix addition of -liconv in gdlib.pc.in

package/gd: fix addition of -liconv in gdlib.pc.in

Static build of gnuplot with gd and libiconv is broken since bump to
version 2.3.1 in commit 970b2ca3cc3f927f679c871eeadb22ec110b0ed5:

/home/giuliobenetti/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/powerpc-buildroot-linux-uclibc/9.3.0/../../../../powerpc-buildroot-linux-uclibc/bin/ld: /home/giuliobenetti/autobuild/run/instance-3/output-1/host/bin/../powerpc-buildroot-linux-uclibc/sysroot/usr/lib/libgd.a(gdkanji.o): in function `do_convert':
gdkanji.c:(.text+0x148): undefined reference to `libiconv_open'
/home/giuliobenetti/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/powerpc-buildroot-linux-uclibc/9.3.0/../../../../powerpc-buildroot-linux-uclibc/bin/ld: gdkanji.c:(.text+0x1d0): undefined reference to `libiconv'

This build failure is raised because LIBS has been replaced by
LIBS_PRIVATES in gdlib.pc.in since
https://github.com/libgd/libgd/commit/28ecfe77c817aff8ce56422d3e4e8533a281bc76

Fixes:
 - http://autobuild.buildroot.org/results/5ab5f4744adfd8d8be483204a9c7f59e34ce26c6

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