qtwebengine: Add missing libxkbfile dependency
qtwebengine: Add missing libxkbfile dependency

The libxkbfile is a new dependency of qtwebengine 5.15.8 updated in commit
d38470c2 ("qtwebengine: upgrade to v5.15.8 and use the same SRCREVs as qtpdf")
Without libxkbfile dependency, the build fails with:

```
sed: can't read /.../qtwebengine/5.15.8+gitAUTOINC+73e76f9e86_48a205f9e0-r0/image/usr/lib/pkgconfig/Qt5WebEngineCore.pc: No such file or directory
```

The sed failure is fatal, but doesn't show what's wrong,
log.do_configure shows it nicely:

```
  QtWebEngine required system libraries for qpa-xcb:
    x11 .................................. yes
    libdrm ............................... yes
    xcomposite ........................... yes
    xcursor .............................. yes
    xi ................................... yes
    xproto (glproto) ..................... yes
    xtst ................................. yes
    xkbfile .............................. no
...
Note: The following modules are not being compiled in this
configuration:
    webenginecore
    webengine
    webenginewidgets

WARNING: Could not find all necessary libraries for qpa-xcb support in
QtWebEngine.

WARNING: QtWebEngine will not be built.
```

And it happens only with x11 in DISTRO_FEATURES <- that's why I wasn't
seeing it in my builds before.

Signed-off-by: Marek Vasut <f40600653b33f5282fa4c5daf377dfc444fb4625@denx.de>
Signed-off-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>

meta-qt5/meta-qt5,meta-qt5/meta-qt5