install_prerequisites.sh: install debhelper apt package

Add debhelper to the list of apt packages to install in order to support
building debian packages.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I50c8bead1b0a0281bfd8ec0aa2b0464a99bb57cb