compiler.h: Introduce __must_be_byte_array()

In preparation for adding stricter type checking to the str/mem*()
helpers, provide a way to check that a variable is a byte array
via __must_be_byte_array().

Suggested-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Kees Cook <kees@kernel.org>