gallium: hud_context: Fix scalar initializer warning.

gallium: hud_context: Fix scalar initializer warning.

When trying to build mesa/master under AOSP, I've run into the
following error:

external/mesa3d/src/gallium/auxiliary/hud/hud_context.c:1821:31: error: braces around scalar initializer [-Werror,-Wbraced-scalar-init]
   struct sigaction action = {{0}};
                              ^~~
1 error generated.

This patch addresses this by switching to using memset instead of
using an initializer.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4141>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4141>