fix: file a slug-platform bios under its own system

install/retrodeck.json

@@ -75482,6 +75482,9 @@
       "cores": [
         "picodrive"
       ],
+      "systems": [
+        "sega-mega-cd"
+      ],
       "regions": [
         "north-america"
       ],
@@ -75499,6 +75502,9 @@
       "cores": [
         "picodrive"
       ],
+      "systems": [
+        "sega-mega-cd"
+      ],
       "regions": [
         "north-america"
       ],
@@ -75515,6 +75521,9 @@
       "repo_path": "bios/Sega/Mega CD/bios_CD_U.bin",
       "cores": [
         "picodrive"
+      ],
+      "systems": [
+        "sega-mega-cd"
       ]
     },
     {