configure.ac: append to existing initializer override flags

Currently we were overwriting the existing warning flags, instead of
adding new [as applicable].

Fixes c5d2e2d43f6 ("configure: Test for -Wno-initializer-overrides")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
1 file changed