app.h: Restore C++ compatibility for macro AST_DECLARE_APP_ARGS

This partially reverts commit 3d1bf3c5,
specifically for app.h.

This works with both gcc 9.3.0 and 10.2.0 now, both for C and C++ (as
tested with external modules).

ASTERISK-29287

Change-Id: I5b9f02a9b290675682a1d13f1788fdda597c9fca
Signed-off-by: Jaco Kroon <jaco@uls.co.za>