[FS#1371] uml: fails to build with musl (fix inside)

[LEDE Bugs]

The following task has a new comment added:

FS#1371 - uml: fails to build with musl (fix inside)
User who did this - Mathias Kresin (mkresin)

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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1371#comment4351