basic: don't link "libm.so" into "libbasic.so" (#5628)

Very few parts of the systemd source require <math.h> or "libm.so".
Linking libbasic with -lm drags the mathematical library in for all
systemd components, and in turn for all users of systemd libraries.

It's just unneeded.
(cherry picked from commit 1539a651)