cmake: add version to the shared library

[spvkgn]

If planned to add to major Linux distros, the shared library should be versioned, i.e. as described in Debian policy.

$ tree
.
├── include
│   └── ultrahdr_api.h
└── lib
    ├── libuhdr.so -> libuhdr.so.1
    ├── libuhdr.so.1 -> libuhdr.so.1.0
    ├── libuhdr.so.1.0
    └── pkgconfig
        └── libuhdr.pc

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