osmo-ctrl2cgi.service: Fix exec path not absolute

systemd errors due to executabl path not being absolute:
systemd[1]: [/lib/systemd/system/osmo-ctrl2cgi.service:7] Executable path is not absolute, ignoring: python3 /usr/bin/ctrl2cgi.py
systemd[1]: osmo-ctrl2cgi.service: Service lacks both ExecStart= and ExecStop= setting. Refusing.

Change-Id: I8e3d695951fb36d53c112b6dce10a275b757ca5a
diff --git a/contrib/systemd/osmo-ctrl2cgi.service b/contrib/systemd/osmo-ctrl2cgi.service
index 36ff78b..8563c66 100644
--- a/contrib/systemd/osmo-ctrl2cgi.service
+++ b/contrib/systemd/osmo-ctrl2cgi.service
@@ -4,7 +4,7 @@
 [Service]
 Type=simple
 Restart=always
-ExecStart=python3 /usr/bin/ctrl2cgi.py -o -d -c /etc/osmocom/ctrl2cgi.ini
+ExecStart=/usr/bin/ctrl2cgi.py -o -d -c /etc/osmocom/ctrl2cgi.ini
 RestartSec=2
 
 [Install]