Fix: protect visibility of filter-parser functions

Some functions share the same name between ctf-parser/lexer and
filter-parser/lexer.

This patch protects the visibility of the functions with the same name
to avoid linkage confusions.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>