package/localedef: depend host build on python3

package/localedef: depend host build on python3

Just like glibc, host-localedef needs python3 on the host to
build... since host-localedef is basically using the sources of glibc.

Fixes:

checking if /build/build/per-package/host-localedef/host/bin/ccache
/usr/bin/gcc is sufficient to build libc... yes
checking for x86_64-pc-linux-gnu-nm... /usr/bin/nm
checking for python3... no
checking for python... python
checking version of python... 2.7.18, bad
configure: error:
*** These critical programs are missing or too old: python
*** Check the INSTALL file for required versions.

As reported at:

  http://lists.busybox.net/pipermail/buildroot/2020-September/291929.html

Signed-off-by: Michael Nosthoff <buildroot@heine.tech>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>