gitlab-ci: Remove -lm from serd-0.pc.

7 jobs from master in 3 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
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passed #84727591
arm-linux-gnueabihf

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passed #84727590
i686-linux-gnu

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passed #84727593
i686-w64-mingw32

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passed #84727594
universal-apple-darwin

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passed #84727589
x86_64-linux-gnu

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passed #84727592
x86_64-w64-mingw32

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