pid1: do not fail if dlopen of libmount fails because it is not compiled in

It is now possible to build and run without libmount. But if support
is compiled in, then we need to be able to load it. Hence gracefully
skip it when support is not compiled in, but fail otherwise.

src/core/main.c

@@ -3311,8 +3311,9 @@ int main(int argc, char *argv[]) {
                 goto finish;
         }
 
+        /* Building without libmount is allowed, but if it is compiled in, then we must be able to load it */
         r = dlopen_libmount();
-        if (r < 0) {
+        if (r < 0 && !ERRNO_IS_NEG_NOT_SUPPORTED(r)) {
                 error_message = "Failed to load libmount.so";
                 goto finish;
         }