fix ncnn example find ncnn package (#282)

Author: hanrui1sensetime

cmake/MMDeployConfig.cmake.in

@@ -33,6 +33,9 @@ endif()
 if ("ort" IN_LIST MMDEPLOY_TARGET_BACKENDS)
     find_package(ONNXRUNTIME)
 endif()
+if ("ncnn" IN_LIST MMDEPLOY_TARGET_BACKENDS)
+    find_package(ncnn REQUIRED)
+endif()
 list(POP_BACK CMAKE_MODULE_PATH)
 
 find_package(spdlog REQUIRED)

docs/en/build/android.md

@@ -198,6 +198,7 @@ mkdir -p build && cd build
 cmake .. \
       -DOpenCV_DIR=${OPENCV_ANDROID_SDK_DIR}/sdk/native/jni/abi-arm64-v8a \
       -Dspdlog_DIR=${SPDLOG_DIR}/lib/cmake/spdlog \
+      -Dncnn_DIR=${NCNN_DIR}/build/install/lib/cmake/ncnn \
       -DMMDeploy_DIR=${MMDEPLOY_DIR}/build/install/lib/cmake/MMDeploy \
       -DCMAKE_TOOLCHAIN_FILE=${NDK_PATH}/build/cmake/android.toolchain.cmake \
       -DANDROID_ABI=arm64-v8a \

docs/zh_cn/build/android.md

@@ -193,6 +193,7 @@ mkdir -p build && cd build
 cmake .. \
       -DOpenCV_DIR=${OPENCV_ANDROID_SDK_DIR}/sdk/native/jni/abi-arm64-v8a \
       -Dspdlog_DIR=${SPDLOG_DIR}/lib/cmake/spdlog \
+      -Dncnn_DIR=${NCNN_DIR}/build/install/lib/cmake/ncnn \
       -DMMDeploy_DIR=${MMDEPLOY_DIR}/build/install/lib/cmake/MMDeploy \
       -DCMAKE_TOOLCHAIN_FILE=${NDK_PATH}/build/cmake/android.toolchain.cmake \
       -DANDROID_ABI=arm64-v8a \