RPM: Add missing BuildRequires for PAM component

When the optional PAM binaries are included in a build, ./configure will
look for security/pam_modules.h and - if it doesn't find it - recommend
the user install libpam0g-dev. On Red Hat systems, pam-devel is the
package that supplies this requirement; libpam0g-dev does not exist.

By encoding this requirement in the spec file, we give packagers more
appropriate (and timely) recommendations for completing the build.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Lindee <chris.lindee+github@gmail.com>
Closes #13001