ci: Add Werror=misleading-indentation to debian-clang

Due to some issues with GCC and this warning in very long files, we
disabled it when compiling NIR.  Unfortunately by design Meson doesn't
allow us to set flags per source file.

The warning is still enabled in clang. but it is less commonly
used during development.  To avoid missing catching those warnings,
add -Werror=misleading-indentation to the GitLab CI debian-clang build.

See https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25315 for
more context.  This patch is a transcription of what Eric Engestrom
suggested, except only targetting C flags (since we only disable them
for C in NIR build).

Acked-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26938>

.gitlab-ci/build/gitlab-ci.yml

@@ -469,6 +469,7 @@ debian-clang:
       -Wno-error=enum-conversion
       -Wno-error=initializer-overrides
       -Wno-error=sometimes-uninitialized
+      -Werror=misleading-indentation
     CPP_ARGS: >
       -Wno-error=c99-designator
       -Wno-error=overloaded-virtual