handy.h: Fix isBLANK_LC_uni()

handy.h: Fix isBLANK_LC_uni()

This macro can be defined in terms of the foo_uvchr() version.  It
should be correct on platforms that have an isblank() function in the C
library.  I don't know why this macro exists.  It doesn't correspond to
any of the other ones (though a recent commit removed one it did
correspond to, but which can't have been in use because it expanded to a
non-existent function).  I'm leaving it in just for back compat.  I did
not add tests for this macro.