Fix 'visibility' attribute ignored warnings.

This commit removes the WEBVTT_INTERN decorator from static functions.

WEBVTT_INTERN sets the visibility("hidden") attribute on GCC 4
and later. Functions declared static already have hidden linkage
and gcc 4.7.2 -Wattributes warns about the redundant attribute being
ignored.

According to http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
"The visibility attribute should be applied only to declarations that
would otherwise have external linkage."