build: Fixed Visual Studio project file generation of strdup.[c|h]

build: Fixed Visual Studio project file generation of strdup.[c|h]

As the curl command-line tool now includes it's own version of strdup(),
for platforms that don't have it, fixed up the git respository Visual
Studio project file generator to not include the version from lib in the
tool project files, rather than having both lib\strdup.[c|h] and
src\tool_strdup.[c|h] present.