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

MANIFEST.usm

@@ -24,8 +24,7 @@
       "lib:libinversion-0.1.so",
       "lib:libastralis-0.1.so",
       "lib:libsodium.so.26",
-      "lib:libxml2.so.2",
-      "lib:libc.so.6"
+      "lib:libxml2.so.2"
     ],
     "build": [
       "bin:valac",