Fix MacOS Posix port (#957)

PR #914 caused Posix Port to fail to build on MacOS. This PR fixes
teh build failure.

This PR also adds a Matrix configuration to the GitHub kernel-demo
workflow to build the Posix Demos on MacOS.
---------

Co-authored-by: chinglee-iot <61685396+chinglee-iot@users.noreply.github.com>
Co-authored-by: Gaurav-Aggarwal-AWS <33462878+aggarg@users.noreply.github.com>

.github/workflows/kernel-demos.yml

@@ -65,7 +65,13 @@ jobs:
 
   POSIX-GCC:
     name: Native GCC
-    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - macos-latest
+          - ubuntu-latest
+    runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout the FreeRTOS/FreeRTOS Repository
         uses: actions/checkout@v3
@@ -83,6 +89,7 @@ jobs:
 
       - name: Install GCC
         shell: bash
+        if: matrix.os == 'ubuntu-latest'
         run: |
           sudo apt-get -y update
           sudo apt-get -y install build-essential