Install libxml2-devel

Fixed: When installing on openSUSE/Tumbleweed, install.sh fails for lack of libxml2-devel
#978

Author: teusbenschop

help/install.sh

@@ -180,6 +180,7 @@ zypper -n --non-interactive --no-gpg-checks install curl
 zypper -n --non-interactive --no-gpg-checks install make
 zypper -n --non-interactive --no-gpg-checks install utf8proc-devel
 zypper -n --non-interactive --no-gpg-checks install pugixml-devel
+zypper -n --non-interactive --no-gpg-checks install libxml2-devel
 fi