shared/install: in install_context_mark_for_removal ignore not found units

With the following test case:
[Install]
WantedBy= default.target
Also=foobar-unknown.service

disabling would fail with:
$ ./systemctl --root=/ disable testing.service
Cannot find unit foobar-unknown.service.       # this is level debug
Failed to disable: No such file or directory.  # this is the error

After the change we proceed:
$ ./systemctl --root=/ disable testing.service
Cannot find unit foobar-unknown.service.
Removed /etc/systemd/system/default.target.wants/testing.service.

This does not affect specifying a missing unit directly:
$ ./systemctl --root=/ disable nosuch.service
Failed to disable: No such file or directory.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/shared/install.h b/src/shared/install.h
index c6aa4f6ef1..b1f220693b 100644
--- a/src/shared/install.h
+++ b/src/shared/install.h
@@ -119,10 +119,10 @@ struct UnitFileInstallInfo {
         char **also;
 
         char *default_instance;
+        char *symlink_target;
 
         UnitFileType type;
-
-        char *symlink_target;
+        bool auxiliary;
 };
 
 static inline bool UNIT_FILE_INSTALL_INFO_HAS_RULES(UnitFileInstallInfo *i) {