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

MANIFEST.usm

@@ -15,9 +15,7 @@
   "depends": {
     "runtime": [
       "lib:libglib-2.0.so.0",
-      "lib:libgobject-2.0.so.0",
-      "lib:libm.so.6",
-      "lib:libc.so.6"
+      "lib:libgobject-2.0.so.0"
     ],
     "build": [
       "bin:valac",