Install libyang to azure pipeline (#17)

#### Why I did it
sonic-swss-common lib will add dependency to libyang soon, so need install libyang lib to prevent build and UT break.

#### How I did it
Modify azure pipeline to install libyang in azure pipeline steps.

#### How to verify it
Pass all UT.

#### Which release branch to backport (provide reason below if selected)

#### Description for the changelog
Modify azure pipeline to install libyang in azure pipeline steps.

#### Link to config_db schema for YANG module changes

#### A picture of a cute animal (not mandatory but encouraged)

Author: liuh-80

.azure-pipelines/build.yml

@@ -47,6 +47,27 @@ jobs:
   - checkout: self
     clean: true
     submodules: true
+  - task: DownloadPipelineArtifact@2
+    inputs:
+      source: specific
+      project: build
+      pipeline: Azure.sonic-buildimage.common_libs
+      runVersion: 'latestFromBranch'
+      runBranch: 'refs/heads/master'
+      path: $(Build.ArtifactStagingDirectory)/download
+      ${{ if eq(parameters.arch, 'amd64') }}:
+        artifact: common-lib
+      ${{ else }}:
+        artifact: common-lib.${{ parameters.arch }}
+      patterns: |
+        target/debs/buster/libyang-*.deb
+        target/debs/buster/libyang_*.deb
+    displayName: "Download libyang from common lib"
+  - script: |
+      set -ex
+      sudo dpkg -i $(find ./download -name *.deb)
+    workingDirectory: $(Build.ArtifactStagingDirectory)
+    displayName: "Install libyang from common lib"
   - task: DownloadPipelineArtifact@2
     inputs:
       source: specific

lgtm.yml

@@ -15,6 +15,8 @@ extraction:
       - "swig3.0"
       - "uuid-dev"
       - "libzmq3-dev"
+      - "libyang"
+      - "libyang-dev"
     after_prepare:
     - "git clone https://github.com/Azure/sonic-swss-common; pushd sonic-swss-common; ./autogen.sh; fakeroot dpkg-buildpackage -us -uc -b; popd"
     - "dpkg-deb -x libswsscommon_1.0.0_amd64.deb $LGTM_WORKSPACE"