coreutils: fix rename-with-trailing-slash bug override variable name

The variable controlling if coreutils thinks the system has the
rename-with-trailing-slash bug is called
gl_cv_func_rename_trailing_slash_bug and not vb_cv_..

Forcing this off works around a bug in coreutils configure, which
otherwise tries to compile Windows-only workaround code.

Reported-by: Stephen Rodgers <hwstar@rodgers.sdcoxmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>