Make wasi-nn backends as separated shared libraries (#3509)

- All files under *core/iwasm/libraries/wasi-nn* are compiled as shared libraries
- *wasi-nn.c* is shared between backends
- Every backend has a separated shared library
- If wasi-nn feature is enabled, iwasm will depend on shared library libiwasm.so
  instead of linking static library libvmlib.a

core/iwasm/interpreter/wasm_runtime.h

@@ -13,10 +13,6 @@
 #include "../common/wasm_runtime_common.h"
 #include "../common/wasm_exec_env.h"
 
-#if WASM_ENABLE_WASI_NN != 0
-#include "../libraries/wasi-nn/src/wasi_nn_private.h"
-#endif
-
 #ifdef __cplusplus
 extern "C" {
 #endif