Update meson build with completions

[fedang]

the use of file names instead of some glob seems silly, but I couldn't figure out a smarter way

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.02%. Comparing base (fdcda3d) to head (e87899e).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1511   +/-   ##
=======================================
  Coverage   65.02%   65.02%           
=======================================
  Files          51       51           
  Lines        9129     9129           
  Branches     1064     1064           
=======================================
  Hits         5936     5936           
  Misses       3193     3193           

Flag	Coverage Δ
unittests	65.02% <ø> (ø)

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[CelDaemon]

the use of file names instead of some glob seems silly, but I couldn't figure out a smarter way

A smarter way isn't needed, meson doesn't support glob for performance reasons and recommends manually specifying files.
The same thing is also done in the rest of the project, like in src/meson.build:

dunst/src/meson.build

Lines 1 to 19 in fdcda3d

	dunst_src_files = files(
	'dbus.c',
	'draw.c',
	'dunst.c',
	'icon-lookup.c',
	'icon.c',
	'ini.c',
	'input.c',
	'log.c',
	'markup.c',
	'menu.c',
	'notification.c',
	'option_parser.c',
	'output.c',
	'queues.c',
	'rules.c',
	'settings.c',
	'utils.c',
	)