Undefine av_always_inline before setting it to an empty value.

This avoids a ton of redefinition warnings.
patch by Jeff Downs, heydowns borg com

Originally committed as revision 19548 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2655,6 +2655,7 @@ print_config CONFIG_ $TMPH config.mak $CONFIG_LIST       \
 echo "#define restrict $_restrict" >> $TMPH
 
 if enabled small; then
+    echo "#undef av_always_inline"  >> $TMPH
     echo "#define av_always_inline"  >> $TMPH
 fi