UI (simple_dialog): fix warning: redundant redeclaration of .. [-Wredundant-decls]

simple_dialog.h:78:13: warning: redundant redeclaration of ‘simple_message_box’ [-Wredundant-decls]
simple_dialog.h:85:13: warning: redundant redeclaration of ‘vsimple_error_message_box’ [-Wredundant-decls]
simple_dialog.h:90:13: warning: redundant redeclaration of ‘simple_error_message_box’ [-Wredundant-decls]

Change-Id: I8f6363fc8ec46ba24895e5236368544f64354e42
Reviewed-on: https://code.wireshark.org/review/4460
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>