Fix format checker to exclude git submodules

This replaces 'find' with 'git ls-files' to avoid checking source files
from submodules like portaudio, mini-gdbstub, and minislirp.

Root cause: The find command recursively searches the entire directory
tree, including initialized submodules. This causes CI failures when
third-party code does not match the project's formatting standards.

Author: jserv

.ci/check-format.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-SOURCES=$(find $(git rev-parse --show-toplevel) | egrep "\.(c|cxx|cpp|h|hpp)\$")
+SOURCES=$(git ls-files '*.c' '*.cxx' '*.cpp' '*.h' '*.hpp')
 
 set -x