Add libmodem to name-allowlist

Allows the use of:
west config manifest.group-filter -- +libmodem

Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>
(cherry picked from commit 81322a7)

Author: MarkusLassila

west.yml

@@ -20,6 +20,7 @@ manifest:
           - cmsis_6
           - cmock
           - hal_nordic
+          - libmodem
           - mbedtls
           - mcuboot
           - memfault-firmware-sdk