Add -lm to the audiod ldflags.

Add -lm to the audiod ldflags.

Depending on the set of installed (optional) libraries, para_audiod gets or
does not get automatically linked against the math library. However, we always
need this lib as the wma decoder is always supported and uses trigonometric
functions.

If -lm is not automatically included, linking fails with many errors of the form

	imdct.c:79: error: 'cos_16' undeclared here (not in a function)

Fix this problem by explicitely adding -lm to audiod's ldflags, just as we
do for para_filter as well.