install_prereq: Fix check_installed_debs remove subversion

check_installed_debs wasn't handling virtual packages like libsrtp-dev and
libresample-dev and on multiarch systems it was accidentally filtering out all
packages if any :i386 packages were found instead of just filtering out the
:i386 packages themselves.

Change-Id: Ifd68da0d1ee30cc84df14de3f9b9079d7c3cecda

contrib/scripts/install_prereq

@@ -66,13 +66,12 @@ in_test_mode() {
 }
 
 check_installed_debs() {
-	for pack in "$@"
-	do
-		tocheck="${tocheck} ^${pack}$"
+	for pack in "$@" ; do
+		tocheck="${tocheck} ^${pack}$ ~P^${pack}$"
 	done
 	pkgs=$(aptitude -F '%c %p' search ${tocheck} 2>/dev/null | awk '/^p/{print $2}')
-	if ! [ ${#pkgs} -eq 0 ]; then
-		echo $pkgs | grep -v ':i386$'
+	if [ ${#pkgs} -ne 0 ]; then
+		echo $pkgs | sed -r -e "s/ ?[^ :]+:i386//g"
 	fi
 }