Add openssl include path for sockets_posix target

[gmtt]

Issue #, if available:
When compile device sdk as shared library, compiler complaining openssl/ssl.h header file not found, in the cmakelists.txt, it has find_package(OpenSSL), but the OPENSSL_INCLUDE_DIR is not used anywhere, it only linked OPENSSL_LIBRARIES

The cmake configure and compile command I used:

cmake -DCMAKE_TOOLCHAIN_FILE=$IOTMI_TOOLCHAIN_PATH/toolchain.cmake \                                                                                                                                            
  -DCMAKE_INSTALL_PREFIX=$SYSROOT_PATH/usr/local \
  -DCMAKE_PREFIX_PATH=$SYSROOT_PATH/usr/local \
  -DCMAKE_BUILD_TYPE=Release \
  -DBUILD_DEMOS=OFF \
  -DBUILD_CLONE_SUBMODULES=OFF \
  -DDOWNLOAD_CERTS=OFF \
  -DINSTALL_TO_SYSTEM=ON \
  -DBUILD_SHARED_LIBS=ON \
  ..

cmake --build . --target install

Description of changes:
Add OPENSSL_INCLUDE_DIR to include directories for target sockets_posix

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