package/gd: fix build when libiconv is enabled

First of two patches to fix
http://autobuild.buildroot.net/results/238/2386edb7f95920e84a35811a33f4333ee0a7a860/



gd links against libiconv if it is already built, depend on libiconv
to get reproducable builds.

readelf output without libiconv present:
$ output/host/opt/ext-toolchain/bfin-linux-uclibc/bin/bfin-linux-uclibc-readelf \
  -a output/staging/usr/lib/libgd.a | grep iconv
    15: 00000000    12 FUNC    GLOBAL HIDDEN     1 _iconv_open
    16: 0000000c    12 FUNC    GLOBAL HIDDEN     1 _iconv
    17: 00000018    12 FUNC    GLOBAL HIDDEN     1 _iconv_close

readelf output with libiconv present:
$ output/host/opt/ext-toolchain/bfin-linux-uclibc/bin/bfin-linux-uclibc-readelf \
  -a output/staging/usr/lib/libgd.a | grep iconv
000000e4  0000100a R_BFIN_PCREL24    00000000   _libiconv_open + 0
00000140  0000140a R_BFIN_PCREL24    00000000   _libiconv + 0
0000019a  0000160a R_BFIN_PCREL24    00000000   _libiconv_close + 0
    16: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND _libiconv_open
    20: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND _libiconv
    22: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND _libiconv_close

[Peter: also add to LIBS so it ends up in gdlib-config --libs output]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>