Update wpebackend-fdo to 1.6.1

[Adrian Perez de Castro]

Impact
------

This is a minor bugfix release of the 1.6.x series:

  https://wpewebkit.org/release/wpebackend-fdo-1.6.1.html

The version currently in 20.04 LTS is 1.6.0, which accidentally shipped with
a bug that made the symbols wpe_fdo_get_{major,minor,micro}_version hidden.

These were supposed to be public (which is fixed in 1.6.1) and can be used
by other components to check the installed version at runtime.

In particular WebKitGTK depends on wpebackend-fdo, and we expect future
security updates may use these functions. The WebKitGTK team has committed
to making it possible for 20.04 to get updates until April 2023, and this
update makes those possible. Users of WebKit-based browsers should be able
to browse the Web with an up-to-date WebKitGTK stack that receives security
updates :)

Test Plan
---------

The following C program can be used to check that the bug is present before
the wpebackend-fdo update, and that the issue was fixed afterwards:

  /*
   * $CC -o wpefdoversion wpefdoversion.c $(pkg-config --cflags --libs wpebackend-fdo-1.0)
   *
   * This will:
   * - Fail to link with wpebackend-fdo 1.6.0
   * - Succeed with version 1.6.1
   */

  #include <wpe/fdo.h>
  #include <stdio.h>

  int main(int argc, char *argv[]) {
    printf("wpebackend-fdo runtime version: %d.%d.%d\n",
           wpe_fdo_get_major_version(), wpe_fdo_get_minor_version(), wpe_fdo_get_micro_version());
    return 0;
  }

Browsers which use WebKitGTK for rendering web content running in a Wayland session
use wpebackend-fdo. Loading e.g. https://people.igalia.com/aperez/poster-circle.html
in Epiphany (package epiphany-browser) should work the same before and after the update.

What Could Go Wrong
-------------------

Given the little amount of changes between versions 1.6.0 and 1.6.1, the possibility
of breakage is minimal to none. Diff:

  https://github.com/Igalia/WPEBackend-fdo/compare/1.6.0...1.6.1

If there were unexpected build failures for the package, an alternative would be to
continue shipping version 1.6.0 with commit 1dda80de5372e56cdc6f818a3fca493f7cc1f9d6
applied, which would solve the issue with the wpe_fdo_get_{major,minor,micro}_version
symbols without including the rest of the changes from 1.6.1:

  https://github.com/Igalia/WPEBackend-fdo/commit/1dda80de5372e56cdc6f818a3fca493f7cc1f9d6

Finally, given that default installs of Ubuntu desktop do not include a WebKitGTK
based browser, users are likely to have another web browser available if things were
to go wrong.