Fix the following build error with meson:
/usr/bin/python3 /home/bgalvani/work/NetworkManager/tools/generate-docs-nm-settings-docs-merge.py man/nm-settings-docs-nmcli.xml --only-from-first clients/cli/generate-docs-nm-settings-nmcli.xml libnm/nm-propery-infos-nmcli.xml libnm/nm-settings-docs-gir.xml
Traceback (most recent call last):
File "/home/bgalvani/work/NetworkManager/tools/generate-docs-nm-settings-docs-merge.py", line 120, in <module>
xml_roots = list([ET.parse(f).getroot() for f in gl_input_files])
File "/home/bgalvani/work/NetworkManager/tools/generate-docs-nm-settings-docs-merge.py", line 120, in <listcomp>
xml_roots = list([ET.parse(f).getroot() for f in gl_input_files])
File "/usr/lib64/python3.8/xml/etree/ElementTree.py", line 1202, in parse
tree.parse(source, parser)
File "/usr/lib64/python3.8/xml/etree/ElementTree.py", line 584, in parse
source = open(source, "rb")
FileNotFoundError: [Errno 2] No such file or directory: '--only-from-first'
Fixes: 3c11116c48 ('docs: in "generate-docs-nm-settings-docs-merge.py" only take properties from first setting')