liblzma: Prevent an empty translation unit in Windows builds.

liblzma: Prevent an empty translation unit in Windows builds.

To workaround Automake lacking Windows resource compiler support, an
empty source file is compiled to overwrite the resource files for static
library builds. Translation units without an external declaration are
not allowed by the C standard and result in a warning when used with
-Wempty-translation-unit (Clang) or -pedantic (GCC).

(cherry picked from commit 19899340cf74d98304f9f5b726c72e85c7017d72)