MT#61751 Install systemd service file in /usr

Fixes lintian issues reported on Debian/trixie (lintian v2.121.1):

| E: ngcp-sems: aliased-location [lib/]
| E: ngcp-sems: aliased-location [lib/systemd/]
| E: ngcp-sems: aliased-location [lib/systemd/system/]
| E: ngcp-sems: aliased-location [lib/systemd/system/sems.service]

FYI:

| % lintian-explain-tags aliased-location
| N:
| E: aliased-location
| N:
| N:   This package installs files into an aliased location and should not be doing so.
| N:
| N:   Since Debian Trixie, the base-files package sets up symbolic links such as /bin pointing to usr/bin. Installing files in /bin directly thus triggers undefined behaviour in dpkg.
| N:
| N:   Packages must no longer install files into such locations and must install them to the corresponding location under usr/ instead.
| N:
| N:   Please refer to Binaries (Section 10.1) in the Debian Policy Manual and Bug#1074014 for details.
| N:
| N:   Visibility: error
| N:   Show-Always: no
| N:   Check: files/hierarchy/standard
| N:

FTR, it's also /usr/lib/systemd/system/ngcp-sems.service
already on our Debian/bookworm based systems:

| root@sp1:~# lsb_release -c -s
| No LSB modules are available.
| bookworm
|
| root@sp1:~# ls -ld /lib /usr
| lrwxrwxrwx  1 root root    7 Dec 12 01:08 /lib -> usr/lib
| drwxr-xr-x 12 root root 4096 Dec 12 01:08 /usr
|
| root@sp1:~# realpath /lib/systemd/system/sems.service
| /usr/lib/systemd/system/ngcp-sems.service

Change-Id: I70cd4db2d3fa449e9d75ecb427fd08648bb0d4eb

debian/ngcp-sems.links

@@ -1 +1 @@
-lib/systemd/system/ngcp-sems.service lib/systemd/system/sems.service
+usr/lib/systemd/system/ngcp-sems.service usr/lib/systemd/system/sems.service

debian/ngcp-sems.lintian-overrides

@@ -1,3 +1,3 @@
 ngcp-sems: possible-gpl-code-linked-with-openssl
 ngcp-sems: init.d-script-not-included-in-package etc/init.d/ngcp-sems
-ngcp-sems: service-file-is-not-a-file [lib/systemd/system/sems.service]
+ngcp-sems: service-file-is-not-a-file [usr/lib/systemd/system/sems.service]