src/Makefile.am: Unconditionally link collectd with libm.

Under FreeBSD, running src/utils_format_json.c fails because isfinite()
cannot be resolved. The GNU manpages for isnan() and isfinite() also
advise users to link with libm and AIX has had the library being linked
in for a while. Since it's such a basic library it seems easier to just
link with it unconditionally.

Fixes Github issue #122.