Fix compilation of to signbit call.
In some toolchains, signbit isn't available in <math.h> ('cause it just
includes #include<cmath> instead) and requires std::.
But then, on the ones that supports it, it will not find without #include<cmath>,
which we then need to add too.
Change-Id: I1bb0e2153cc46384ed435149538be5926d4bfca8
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