Fix compilation error on older gcc versions and MSVC.
This fixes an "initializer is not a constant" compilation error that manifests itself on gcc versions < 8.1 and MSVC (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69960#c18).
Fixes bug #40773 (closed)
Signed-off-by: Gabriela Moldovan gabi@torproject.org