winbuild: fix PE version info debug flag

winbuild: fix PE version info debug flag

- Only set PE file flag VS_FF_DEBUG if curl.exe and libcurl.dll were
  built with winbuild option DEBUG=yes which builds with debug info.

VS_FF_DEBUG is a PE flag (Portable Executable file flag - dll, exe, etc)
that indicates the file contains or was built with debug info.

Prior to this change when winbuild was used to build curl, curl.exe
and libcurl.dll always had VS_FF_DEBUG set, regardless of build option
DEBUG=yes/no, due to some bad logic.

Closes https://github.com/curl/curl/pull/13739