Disable deprecation warnings for stable branch

We're not going to replace deprecated functions here.
Signed-off-by: Simon McVittie <smcv@debian.org>
Reviewed-by: Colin Walters <walters@verbum.org>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=98157

configure.ac

@@ -1387,6 +1387,7 @@ TP_COMPILER_WARNINGS([WARNING_CFLAGS],
   dnl - type-limits is probably a bug too, but having the rest of -Wextra
   dnl   is better than nothing
   [$DISABLE_UNUSED_WARNINGS \
+   deprecated-declarations \
    missing-field-initializers \
    unused-parameter \
    pointer-sign \