realtime: Fix exp10 check.

The parameter to exp10() function was constant and because of that it
was substituted by a constant at the compile time.

This patch passes volatile float as the parameter and also adds -lm to
the LIBS which is needed to link the test.

Reported-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>