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>

include/grub/osdep/hostfile_aros.h

@@ -74,7 +74,7 @@ grub_util_readlink (const char *name, char *buf, size_t bufsize)
   return readlink(name, buf, bufsize);
 }
 
-#define grub_util_mkdir(a) mkdir ((a), 0755)
+#define grub_util_mkdir(a) (void) mkdir ((a), 0755)
 
 struct grub_util_fd
 {

include/grub/osdep/hostfile_unix.h

@@ -77,7 +77,7 @@ grub_util_readlink (const char *name, char *buf, size_t bufsize)
   return readlink(name, buf, bufsize);
 }
 
-#define grub_util_mkdir(a) mkdir ((a), 0755)
+#define grub_util_mkdir(a) (void) mkdir ((a), 0755)
 
 #if defined (__NetBSD__)
 /* NetBSD uses /boot for its boot block.  */