Only require qgis.gui import when gui is built

[m-kuhn]

Running qgis_process on a WITH_GUI=OFF build results in the following error

qgis_process run native:buffer
"<font color=\"red\">Couldn't load PyQGIS.<br>Python support will be disabled.</font><br><pre><br>Traceback (most recent call last):<br>&nbsp; File \"<string>\", line 1, in <module><br>ModuleNotFoundError: No module named 'qgis.gui'<br><br></pre>Python version:<br>3.10.12 (main, Jul 29 2024, 16:56:48) [GCC 11.4.0]<br><br>QGIS version:<br>3.34.10-Prizren 'Prizren', 113de9e1<br><br>Python path:<br>['/usr/share/qgis/python', '/home/qgis/.local/share/QGIS/QGIS3/profiles/default/python', '/home/qgis/.local/share/QGIS/QGIS3/profiles/default/python/plugins', '/usr/share/qgis/python/plugins', '/usr/share/qgis/python', '/usr/lib/python310.zip', '/usr/lib/python3.10', '/usr/lib/python3.10/lib-dynload', '/usr/local/lib/python3.10/dist-packages', '/usr/lib/python3/dist-packages']"

Refs opengisch/qgis-slim#3

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit c986d76)