util: Ignore return value for grub_util_mkdir() on all platforms

util: Ignore return value for grub_util_mkdir() on all platforms

Coverity signaled 2 issues where the return value of grub_util_mkdir()
was not being tested.

The Windows variant of this code defines the function as having no
return value (void), but the UNIX variants all are mapped using a macro
to the libc mkdir() function, which returns an int value.

To be consistent, the mapping should cast to void to for these too.

Fixes: CID 73583
Fixes: CID 73617
Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>