install_prereq: Add GMime 3.0.

Ubuntu 20.10 does not come with GMime 2.6. Ubuntu 16.04 LTS does not
come with GMime 3.0. aptitude ignores any missing package. Therefore,
it installs the correct package(s). However, in Ubuntu 18.04 LTS and
Ubuntu 20.04 LTS, both versions are installed alongside although only
one is really needed.

Change-Id: Ic58aa9f2e131d94671f286f17dbd61e1ccbabcb7

contrib/scripts/install_prereq

@@ -24,7 +24,7 @@ PACKAGES_DEBIAN="build-essential pkg-config"
 PACKAGES_DEBIAN="$PACKAGES_DEBIAN libedit-dev libjansson-dev libsqlite3-dev uuid-dev libxml2-dev"
 # Asterisk: for addons:
 PACKAGES_DEBIAN="$PACKAGES_DEBIAN libspeex-dev libspeexdsp-dev libogg-dev libvorbis-dev libasound2-dev portaudio19-dev libcurl4-openssl-dev xmlstarlet bison flex"
-PACKAGES_DEBIAN="$PACKAGES_DEBIAN libpq-dev unixodbc-dev libneon27-dev libgmime-2.6-dev liblua5.2-dev liburiparser-dev libxslt1-dev libssl-dev"
+PACKAGES_DEBIAN="$PACKAGES_DEBIAN libpq-dev unixodbc-dev libneon27-dev libgmime-2.6-dev libgmime-3.0-dev liblua5.2-dev liburiparser-dev libxslt1-dev libssl-dev"
 PACKAGES_DEBIAN="$PACKAGES_DEBIAN libvpb-dev libmysqlclient-dev libbluetooth-dev libradcli-dev freetds-dev libosptk-dev libjack-jackd2-dev bash libcap-dev"
 PACKAGES_DEBIAN="$PACKAGES_DEBIAN libsnmp-dev libiksemel-dev libcorosync-common-dev libcpg-dev libcfg-dev libnewt-dev libpopt-dev libical-dev libspandsp-dev"
 PACKAGES_DEBIAN="$PACKAGES_DEBIAN libresample1-dev libc-client2007e-dev binutils-dev libsrtp0-dev libsrtp2-dev libgsm1-dev doxygen graphviz zlib1g-dev libldap2-dev"