include: move typeof_strip_qual to compiler.h, use it in QAPI_LIST_LENGTH()

include: move typeof_strip_qual to compiler.h, use it in QAPI_LIST_LENGTH()

The typeof_strip_qual() is most useful for the atomic fetch-and-modify
operations in atomic.h, but it can be used elsewhere as well.  For example,
QAPI_LIST_LENGTH() assumes that the argument is not const, which is not a
requirement.

Move the macro to compiler.h and, while at it, move it under #ifndef
__cplusplus to emphasize that it uses C-only constructs.  A C++ version
of typeof_strip_qual() using type traits is possible[1], but beyond the
scope of this patch because the little C++ code that is in QEMU does not
use QAPI.

The patch was tested by changing the declaration of strv_from_str_list()
in qapi/qapi-type-helpers.c to:

    char **strv_from_str_list(const strList *const list)

This is valid C code, and it fails to compile without this change.

[1] https://lore.kernel.org/qemu-devel/20240624205647.112034-1-flwu@google.com/

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Tested-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>