libc: Fix glob.c build with GCC

We don't expose the dummy struct pointer in the header file, so
gl_errblk is just a void * when lacking compiler support for blocks and
we need to cast to the former for CALL_BLOCK to work. If blocks support
is present this should be a redundant cast to the type that gl_errblk
already is.

Fixes: 1e0743f54d2d ("glob: Add blocks support")