package/wavpack: Fix iconv support

package/wavpack: Fix iconv support

wavpack fails to link with libiconv

/bin/bash ../libtool  --tag=CC   --mode=link /home/bernd/buildroot/br3/output/host/usr/bin/i586-buildroot-linux-uclibc-gcc  -I../include -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64   -Os    -o wvunpack wvunpack-wvunpack.o wvunpack-utils.o wvunpack-md5.o  ../src/.libs/libwavpack.la -lm
libtool: link: /home/bernd/buildroot/br3/output/host/usr/bin/i586-buildroot-linux-uclibc-gcc -I../include -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -o wvunpack wvunpack-wvunpack.o wvunpack-utils.o wvunpack-md5.o  ../src/.libs/libwavpack.so -lm -Wl,-rpath -Wl,/home/bernd/buildroot/br3/output/build/wavpack-4.75.2/src/.libs
/home/bernd/buildroot/br3/output/host/usr/lib/gcc/i586-buildroot-linux-uclibc/4.9.3/../../../../i586-buildroot-linux-uclibc/bin/ld: wvunpack-wvunpack.o: undefined reference to symbol 'libiconv_open'

using this defconfig

BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_PACKAGE_WAVPACK=y

Patch suggested by Peter:
http://lists.busybox.net/pipermail/buildroot/2014-February/090596.html

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>