Include errors with compatibility headers in v6-alpha

[rth7680]

https://github.com/capstone-engine/capstone/blob/76242699291a998d27934442ab5b425e2d566a03/include/capstone/arm64.h#L11

and

https://github.com/capstone-engine/capstone/blob/76242699291a998d27934442ab5b425e2d566a03/include/capstone/systemz_compatibility.h#L12

use <capstone/...> to include, whereas everything else uses "".

This fails when using -I$includedir because <> only searches system headers.

Either capstone.pc should use -isystem, or these two instances should also use "".