scripts: gen_compile_commands: fix invalid escape sequence warning

Since Python 3.12 unrecognised escape sequences trigger a SyntaxWarning.
Convert the '\#' string to a raw string so the backslash is correctly
used as a literal.

Ported from Linux commit dae4a0171e25 ("gen_compile_commands: fix invalid
escape sequence warning").

This updates the script to be in-line with Linux 6.9-rc6.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: João Marcos Costa <jmcosta944@gmail.com>