add the include path where ev.h is installed by libev-devel on Centos

The README should probably also be updated with the yum install instructions:
sudo yum install libev libev-devel

The yum install places the header files needed by the new libev wrapper in /usr/include/libev (instead of just /usr/include as done by some other package systems):

repoquery -lq libev-devel
/usr/include/libev
/usr/include/libev/ev++.h
/usr/include/libev/ev.h
/usr/include/libev/event.h
/usr/lib/libev.so
/usr/lib/pkgconfig/libev.pc
/usr/include/libev
/usr/include/libev/ev++.h
/usr/include/libev/ev.h
/usr/include/libev/event.h
/usr/lib64/libev.so
/usr/lib64/pkgconfig/libev.pc

This pull request just adds this as an include path so the build will include the libev wrapper on systems with yum as a package manager (e.g. Centos)