Fix build with panic="abort"

[anassinator]

When building with panic="abort" and std built from source, the size of Mutex<()> varies when wrapped in Option<> or not such that the owned and loaned types have different sizes.

This is specifically due to the poison flag field in Mutex getting compiled out when the panic mode is not set to "unwind". This field's omission pushes the size of the struct to the edge of the 16 byte boundary such that niche optimization no longer has bits to fiddle with after aligning to 8 bytes.

The build fails as follows:

note: evaluation panicked: Size mismatch: type z_loaned_mutex_t has size 16 while type z_owned_mutex_t has size 24

To address, we switch to decl_c_type_inequal! which seems to be safe since we shouldn't be reinterpret_cast()-ing this type and zenoh-cpp doesn't seem to ever reference it.

[github-actions]

PR missing one of the required labels: {'new feature', 'enhancement', 'bug', 'internal', 'breaking-change', 'dependencies', 'documentation'}

[anassinator]

I want second-hand confirmation that this is fixing it the correct way and that there isn't any implicit assumptions in uses of decl_c_type_inequal!() that we're ignoring here.

[DenisBiryukov91]

I want second-hand confirmation that this is fixing it the correct way and that there isn't any implicit assumptions in uses of decl_c_type_inequal!() that we're ignoring here.

Yes, indeed this looks like a correct fix given that z_xxx_mutex is not used in C++ so requirement to have size_of::() == size_of::<Option>() is irrelevant (at least this is similar to what we do with z_owned_mutex_t in zenoh-pico).