Change abs to std::abs

[Veratil]

This prevents GCC 6 from raising an ambiguous call error.

Error reported in Discord: https://discord.com/channels/203559236729438208/207159523616489473/784937509267963904
On Debian 9.13 with GCC 6.3

@PhysSong

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Linux

• Linux (AppImage): lmms-1.3.0-alpha.1.39%2Bg5b4514a-linux-x86_64.AppImage (build link)

Windows

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• Windows 64-bit: lmms-1.3.0-alpha.1.39%2Bg5b4514a92-mingw-win64.exe (build link)
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• Windows 64-bit: lmms-1.3.0-alpha-msvc2017-win64.exe (build link)

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