[url_launcher_linux] Add missing #include

[stuartmorgan-g]

Description

The plugin code uses strcmp, so should include cstring. Currently this
works only due to transitive includes that we can't count on being
present.

Related Issues

None

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[robert-ancell]

I'd normally use <string.h> here as the code is more C-like than C++ like, but otherwise LGTM.