shared/icon-label: Prevent moc from being appended to global file

[gxalpha]

Description

Like described in 5eb04cd (#11101), this prevents the moc_icon-label.cpp from being included in the global mocs_compilation.cpp file. Looks a bit ridiculous to create a new file for this, but that's just because content-wise icon-label is only a header.

Motivation and Context

Oversight from #11148 due to the fact that there was no .cpp file where this include would usually go.

How Has This Been Tested?

macOS 15
Looked at mocs_compilation.cpp, confirmed that <P4W7YBUBWD/moc_icon-label.cpp> was included previously but no longer is.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.