build: stop the librclone tests marking the Linux release binaries as modified

librclone/python/test_rclone.py builds ./librclone.so with
-buildmode=c-shared, which also writes librclone.h. The test removes
the .so but not the header, so the CI Linux job built the release
binaries from a tree with an untracked file and they reported
vcs.modified=true in their build info.

Ignore librclone.h so it no longer counts as a modification.

.gitignore

@@ -19,5 +19,6 @@ Thumbs.db
 __pycache__
 .DS_Store
 resource_windows_*.syso
+/librclone.h
 .devcontainer
 /cmd/bisync/testdata/bisync_vscode_debuggers_launch.json