Auto merge of #24099 - kanaka:fix-gst-top-dir, r=ferjm

Call gstreamer_root with top_dir string, not func.

Commit 72e28dffcdf560543d312abc0ec4ff76b7c7dfaf introduced a build regression on Linux when resolving the in-tree gstreamer libs.

The error:

```
$ ./mach build --release
...
The details of the failure are as follows:

AttributeError: 'function' object has no attribute 'endswith'

  File "/data/joelm/personal/UTA/dissertation/servo/servo-master.git/python/servo/build_commands.py", line 237, in build
    env = self.build_env(target=target, is_build=True)
  File "/data/joelm/personal/UTA/dissertation/servo/servo-master.git/python/servo/command_base.py", line 646, in build_env
    if self.needs_gstreamer_env(target or host_triple(), env):
  File "/data/joelm/personal/UTA/dissertation/servo/servo-master.git/python/servo/command_base.py", line 574, in needs_gstreamer_env
    if path.isdir(gstreamer_root(effective_target, env, self.get_top_dir)):
  File "/data/joelm/personal/UTA/dissertation/servo/servo-master.git/python/servo/command_base.py", line 257, in gstreamer_root
    return path.join(topdir, "support", "linux", "gstreamer", "gst")
  File "/usr/lib/python2.7/posixpath.py", line 70, in join
    elif path == '' or path.endswith('/'):
```

The problem is that get_top_dir is being passed as a function to
gstreamer_root in a couple of places instead of being invoked first.

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