lower pin for libsodium18 pkg

Change-Id: I6d9f7893ddca5c3e08328ef5bfd827a7c709758c
diff --git a/linux/system/repo_local/mcp/extra.yml b/linux/system/repo_local/mcp/extra.yml
index c55ecfc..90339d3 100644
--- a/linux/system/repo_local/mcp/extra.yml
+++ b/linux/system/repo_local/mcp/extra.yml
@@ -15,3 +15,6 @@
           - pin: 'release a=${_param:linux_system_repo_mcp_extra_version}'
             priority: 1100
             package: '*'
+          - pin: 'release a=${_param:linux_system_repo_mcp_extra_version}'
+            priority: 500
+            package: 'libsodium18'