build: add -Wleading-whitespace=spaces

This is available in GCC 15. See
https://gcc.gnu.org/gcc-15/changes.html.

CMakeLists.txt

@@ -463,6 +463,7 @@ else()
   try_append_cxx_flags("-Wself-assign" TARGET warn_interface SKIP_LINK)
   try_append_cxx_flags("-Wbidi-chars=any" TARGET warn_interface SKIP_LINK)
   try_append_cxx_flags("-Wundef" TARGET warn_interface SKIP_LINK)
+  try_append_cxx_flags("-Wleading-whitespace=spaces" TARGET warn_interface SKIP_LINK)
 
   # Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
   # unknown options if any other warning is produced. Test the -Wfoo case, and