dosfstools: fix AM_ICONV triggered build error

dosfstools: fix AM_ICONV triggered build error

Fix this build error:

 gcc -Wall -Wextra -Wno-sign-compare -Wno-missing-field-initializers \
  -Wmissing-prototypes -Wstrict-prototypes -Wwrite-strings -O2 \
  -I/usr/local/src/openwrt/staging_dir/host/include \
  -L/usr/local/src/openwrt/staging_dir/host/lib \
  -o fsck.fat check.o file.o fsck.fat.o lfn.o boot.o common.o fat.o io.o \
  charconv.o @LIBICONV@
 gcc: error: LIBICONV@: No such file or directory
 make[4]: *** [Makefile:449: fsck.fat] Error 1

Suggested-by: Rosen Penev <rosenp@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>