text: Add explicit lifetime to an impl block

While building with rustc <= 1.31 the following error would occur.

error[E0106]: missing lifetime specifier
   --> rsvg_internals/src/text.rs:794:11
    |
794 | impl From<&XmlLang> for pango::Language {
    |           ^ expected lifetime parameter

error: aborting due to previous error

The lifetime is not required with rustc 1.31+ and can be omitted,
but we still need to support older versions for the time being.
7 jobs for <span class="ref-name">alatiera/add-lifetime</span> in 29 minutes and 49 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #179648
fedora test

00:07:15

 
  Lint
manual #179650
allowed to fail manual
clippy
passed #179649
rustfmt

00:01:10

passed #179651
tarball

00:11:46

 
  Cross Distro
passed #179654
debian_i386:testing

00:10:46

passed #179653
debian:testing

00:09:32

passed #179652
fedora:rawhide

00:08:44