Revert the location to install public headers (#4295)

This partly reverts "Cmake improvements".
(https://github.com/bytecodealliance/wasm-micro-runtime/pull/4076)

Recently we changed the location to install public headers.
For example,
Old: include/wasm_export.h
New: include/iwasm/wasm_export.h

For cmake-based user applications using find_package(iwasm),
the cmake package, namely target_include_directories(INSTALL_INTERFACE),
is expected to add necessary compiler options like -isystem automatically.
(See samples/printversion for an example of such user applications.)

However, in reality, not every user application uses cmake.
This commit reverts the location to install public headers
for now, to avoid breakage for non-cmake user applications.

In case we want to re-apply the location change in future,
we should better communicate to the users. (eg. document
migration proceduces in release notes.)

Fixes: https://github.com/bytecodealliance/wasm-micro-runtime/issues/4290

References:
https://cmake.org/cmake/help/latest/prop_tgt/INTERFACE_INCLUDE_DIRECTORIES.html

CMakeLists.txt

@@ -160,7 +160,7 @@ add_library (vmlib ${WAMR_RUNTIME_LIB_SOURCE})
 set_target_properties (vmlib PROPERTIES OUTPUT_NAME iwasm)
 target_include_directories(vmlib INTERFACE
   $<BUILD_INTERFACE:${WAMR_ROOT_DIR}/core/iwasm/include>
-  $<INSTALL_INTERFACE:include/iwasm>
+  $<INSTALL_INTERFACE:include>
 )
 
 target_link_libraries (vmlib PUBLIC ${LLVM_AVAILABLE_LIBS} ${UV_A_LIBS} -lm -ldl ${CMAKE_THREAD_LIBS_INIT})
@@ -189,7 +189,7 @@ set_version_info (vmlib)
 install (TARGETS vmlib
   EXPORT iwasmTargets
   LIBRARY DESTINATION lib
-  PUBLIC_HEADER DESTINATION include/iwasm
+  PUBLIC_HEADER DESTINATION include
 )
 
 install_iwasm_package ()