Add missing #include <cmath> for std::isfinite

[AMDmi3]

Using std::isfinite() requires #include <cmath>, which doesn't exist in surrounding code. This results, for instance, in build failure of ZNC on FreeBSD 11.x (clang 3.8, libc++):

modperl/ZNC.cpp:2573:9: error: no member named 'isfinite' in namespace 'std'; did you mean simply 'isfinite'?
    if (SWIG_Float_Overflow_Check(v)) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
modperl/ZNC.cpp:2561:73: note: expanded from macro 'SWIG_Float_Overflow_Check'
# define SWIG_Float_Overflow_Check(X) ((X < -FLT_MAX || X > FLT_MAX) && SWIG_isfinite(X))
                                                                        ^~~~~~~~~~~~~~~~
modperl/ZNC.cpp:2549:29: note: expanded from macro 'SWIG_isfinite'
#  define SWIG_isfinite(X) (std::isfinite(X))
                            ^~~~~
/usr/include/c++/v1/math.h:380:1: note: 'isfinite' declared here
isfinite(_A1 __lcpp_x) _NOEXCEPT
^
2 warnings and 1 error generated.

full log: http://beefy1.nyi.freebsd.org/data/110i386-quarterly/421620/logs/errors/znc-1.6.3.log

[ojwb]

There's prior discussion in #615.

[wsfulton]

Closing as superceded by #849. Please test it if you can before we release in the day or two!