contrib: cddb: add dependency to gettext

cddb buildsystem, in particular configure.ac, is using macros from gettext.m4, which might not be available on non-GNU systems like the Darwin ones.

When gettext is installed on those systems, the macro file can be imported using the aclocal's -I parameter or using the ACLOCAL_PATH environment variable.

When neither is done, even with gettext in the PATH, the configure.ac won't be able to call AM_ICONV and will trigger the following error.

configure.ac:130: warning: macro 'AM_ICONV' not found in library
autoreconf: configure.ac: tracing
autoreconf: running: libtoolize --copy --force
libtoolize: putting auxiliary files in '.'.
libtoolize: copying file './ltmain.sh'
libtoolize: Consider adding 'AC_CONFIG_MACRO_DIRS([m4])' to configure.ac,
libtoolize: and rerunning libtoolize and aclocal.
libtoolize: Consider adding '-I m4' to ACLOCAL_AMFLAGS in Makefile.am.
configure.ac:130: warning: macro 'AM_ICONV' not found in library
autoreconf: running: /Users/unidan/Workspace/vlc/extras/tools/build/bin/autoconf --include=/Users/unidan/Workspace/vlc/contrib/aarch64-linux-android/share/aclocal --include=/Users/unidan/Workspace/vlc/extras/tools/build/share/aclocal --force
configure.ac:130: error: possibly undefined macro: AM_ICONV
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.

With the gettext contrib, the gettext.m4 will be installed into the directory $(PREFIX)/share/aclocal/gettext.m4, and aclocal will be called with -I $(PREFIX)/share/aclocal/ which solves the issue here.