Add BuildRequires to RHEL specfile

Added a build-time dependency on openssl-devel, so you can easily build
a source RPM and then pass it into mock for building in a clean build
environment (without manually having to install the buildrequires).

Signed-off-by: Jasper Capel <jasper@capel.tv>
Merge pull request #1 from spil-jasper/master

Add BuildRequires to RHEL specfile