libfuse: fix build failures with iconv

Fix build failures when iconv is present for libfuse users that use
pkgconfig.
For example:
http://autobuild.buildroot.net/results/4d455831a7b467b9ea1ad54bea6b57607f5560a8/build-end.log



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>