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

MANIFEST.usm

@@ -22,8 +22,7 @@
       "lib:libinversion-0.1.so",
       "lib:libastralis-0.1.so",
       "lib:libstatum-0.1.so",
-      "lib:libusm-0.1.so",
-      "lib:libc.so.6"
+      "lib:libusm-0.1.so"
     ],
     "build": [
       "bin:valac",