httping: fix static linking when libintl+libiconv are enabled

Fixes http://autobuild.buildroot.net/results/365/365d6be787c5560aa1daa28065668261e1171d24/



If libiconv is enabled, libintl will contain references to it, so we need to
explicitly link against it when statically linking against libintl.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>