cmake: make installed package relocatable

Remove the absolute include header path from the install target. The
created CMake package still has the right include generated:
```cmake
set_target_properties(libdeflate::libdeflate_static PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
)
```

As mentioned in the
(Creating Relocatable Packages)[https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#creating-relocatable-packages]
section of the CMake documentation packages created with
`install(EXPORT)` are designed to be relocatable.

This also makes it possible to cache the install directory over multiple
computer (for example through a package manager).

Author: riegl-gc

CMakeLists.txt

@@ -244,7 +244,7 @@ endif()
 
 set(LIB_INCLUDE_DIRS
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
-    $<INSTALL_INTERFACE:${CMAKE_INSTALL_FULL_INCLUDEDIR}>)
+)
 
 if(LIBDEFLATE_APPLE_FRAMEWORK)
 # Define resource files for Apple Framework