CMakeLists.txt: Support building as shared library (as one would need…

[sunweaver]

… for packaging a DEB package).

[OPNA2608]

The QMake build installs the headers into the equivalent of just CMAKE_INSTALL_INCLUDEDIR. Could we make the install locations match to avoid differences between QMake & CMake being used for the build?

/nix/store/vyqb9vi69yrkmg4j1rw5vsd7rhfq7f5c-qzxing-3.3.0
├── include
│?? ├── QZXing_global.h
│?? └── QZXing.h
└── lib
    ├── libQZXing.so -> libQZXing.so.3.3.0
    ├── libQZXing.so.3 -> libQZXing.so.3.3.0
    ├── libQZXing.so.3.3 -> libQZXing.so.3.3.0
    ├── libQZXing.so.3.3.0
    └── pkgconfig
        └── QZXing.pc

Suggested change

	FILES ${PUBLIC_HEADERS} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/qzxing"
	FILES ${PUBLIC_HEADERS} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"