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

MANIFEST.usm

@@ -24,8 +24,7 @@
       "lib:libxml2.so.2",
       "lib:libz.so.1",
       "lib:libbrotlienc.so.1",
-      "lib:libzstd.so.1",
-      "lib:libc.so.6"
+      "lib:libzstd.so.1"
     ],
     "build": [
       "bin:valac",