motion_est: fix use of inline on extern functions

motion_est: fix use of inline on extern functions

Inline functions declared without extern do not provide an external
definition in standard C99.  This code only works because most
compilers do not implement the inline semantics correctly.  With a
stricter compiler, linking fails with unresolved references to these
functions.

Declaring the functions extern inline works correctly with some
compilers while some others still fail to create external definitions.

For maximum portability, create a static inline version with an
externally visible wrapper for ff_get_mb_score.  ff_epzs_motion_search
is so large that no sane compiler inlines it anyway, so there the
inline keyword can simply be dropped with no effect.

Signed-off-by: Mans Rullgard <mans@mansr.com>