meson: enable the clang-format target

.clang-format needs to exist in the root of the project for the target
to be generated, but since we don't have a global config it's a dummy,
empty file.

.clang-format-include lists the files (files! not folders) that are to
be formatted.

.clang-format-ignore lists the files to exclude, even if they are in the
include list above. Useful for vendored code for instance.

See https://mesonbuild.com/Code-formatting.html

Signed-off-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23269>

.clang-format-include

@@ -0,0 +1,2 @@
+src/**/asahi/**/*
+src/**/panfrost/**/*