avfilter/hermite: fix "libavfilter/hermite.h:19:15: error: no previous prototype for hermite_interpolation"

Fix build

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/hermite.h

@@ -16,7 +16,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-inline double hermite_interpolation(double x, double x0, double x1,
+static inline double hermite_interpolation(double x, double x0, double x1,
                                     double p0, double p1,
                                     double m0, double m1)
 {