[packages] shadow: fix compile

error:
configure.in:592: warning: macro `AM_GNU_GETTEXT_VERSION' not found in library
configure.in:593: warning: macro `AM_GNU_GETTEXT' not found in library
./configure: line 15738: syntax error near unexpected token `0.16'
./configure: line 15738: `AM_GNU_GETTEXT_VERSION(0.16)'

adding dependency

fixes https://dev.openwrt.org/ticket/12004

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>