package/python3: add optional dependency to gettext

package/python3: add optional dependency to gettext

Tested using this defconfig:

BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_SYSTEM_ENABLE_NLS=y
BR2_PACKAGE_PYTHON3=y

Without this patch:
$ readelf -a output/target/usr/lib/libpython3.8.so.1.0 | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libc.so.0]

With this patch:
$ readelf -a output/target/usr/lib/libpython3.8.so.1.0 | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libintl.so.8]
 0x00000001 (NEEDED)                     Shared library: [libc.so.0]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>