enca: fix broken automake

The added patch fixes the usage of AM_ICONV macro and
comes straight from the Buildroot source tree.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>