mkfs.ubifs: Fix build with gcc 5.1

mkfs.ubifs: Fix build with gcc 5.1

In gcc 5.1, the default C standard which is used to compile a C file,
has changed from gnu89 to gnu11. This changed the meaning of 'extern
inline'. See https://gcc.gnu.org/gcc-5/porting_to.html.

In mkfs.ubifs, this leads to multiple definitions of
hashtable_iterator_key and -hashtable_iterator_value. I think the most
pragmatic way to fix the issue is to replace 'extern inline' with
'static inline' here.

Signed-off-by: Bernhard Walle <bernhard@bwalle.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>