Fix for #13252 does not compile on ESR 45
I tried to test our hardened builds setup with the latest branch Arthur posted (15197+11) and the build failed badly pretty early:
```
Reticulating splines...
Traceback (most recent call last):
File "./config.status", line 1061, in <module>
config_status(**args)
File "/home/debian/build/tor-browser/python/mozbuild/mozbuild/config_status.py", line 175, in config_status
definitions = list(definitions)
File "/home/debian/build/tor-browser/python/mozbuild/mozbuild/frontend/emitter.py", line 177, in emit
objs = list(self.emit_from_context(out))
File "/home/debian/build/tor-browser/python/mozbuild/mozbuild/frontend/emitter.py", line 608, in emit_from_context
for obj in self._process_sources(context, passthru):
File "/home/debian/build/tor-browser/python/mozbuild/mozbuild/frontend/emitter.py", line 797, in _process_sources
'exist: \'%s\'' % (symbol, full_path), context)
mozbuild.frontend.reader.SandboxValidationError:
==============================
ERROR PROCESSING MOZBUILD FILE
==============================
The error occurred while processing the following file or one of the files it includes:
/home/debian/build/tor-browser/xpcom/io/moz.build
The error occurred when validating the result of the execution. The reported error is:
File listed in UNIFIED_SOURCES does not exist: '/home/debian/build/tor-browser/xpcom/io/TorFileUtils.cpp'
*** Fix above errors and then restart with\
"make -f client.mk build"
make: *** [configure] Error 1
```
Not sure if that's a rebase issue or something different yet.
issue