Drop libc.so.6 from runtime deps (implicit on Linux, absent on musl)

MANIFEST.usm

@@ -16,8 +16,7 @@
     "runtime": [
       "lib:libglib-2.0.so.0",
       "lib:libgobject-2.0.so.0",
-      "lib:libinvercargill-1.so",
-      "lib:libc.so.6"
+      "lib:libinvercargill-1.so"
     ],
     "build": [
       "bin:valac",