Doxygen generated document did not contain
`poppler::version_string()`, `poppler::version_major()`,
`poppler::version_minor()`, and `poppler::version_micro()`.
These descriptions are written in `poppler-version.h.in`
in source files and doxygen could not parse `poppler-version.h.in`
because its extension is not mapped to any languages.

This commit adds `*.in` to C++ in extension mapping.
As a result, doxygen parses `poppler-version.h.in` as C++ file.
So generated document becomes to contain
`poppler::version_string()` etc.