Add mergesort.c to files to compile in GitHub Actions

This explicitly adds `mergesort.c` to the compile statement for `array_merge` in the GitHub Actions.

Now I need to explain this in the README and make sure they understand what's going on there.

Author: NicMcPhee

.github/workflows/arraymerge_gtest.yml

@@ -33,7 +33,7 @@ jobs:
     - name: Check out the code
       uses: actions/checkout@v2
     - name: Compile test code
-      run: g++ -Wall -g -o array_merge_test array_merge.c array_merge_test.cpp -lgtest -pthread -std=c++0x
+      run: g++ -Wall -g -o array_merge_test ../mergesort/mergesort.c array_merge.c array_merge_test.cpp -lgtest -pthread -std=c++0x
       working-directory: array_merge
     - name: Run test
       run: ./array_merge_test

.github/workflows/arraymerge_test_valgrind.yml

@@ -35,7 +35,7 @@ jobs:
     - name: Check out the code
       uses: actions/checkout@v2
     - name: Compile code
-      run: g++ -Wall -g -o array_merge_test array_merge.c array_merge_test.cpp -lgtest -pthread -std=c++0x
+      run: g++ -Wall -g -o array_merge_test ../mergesort/mergesort.c array_merge.c array_merge_test.cpp -lgtest -pthread -std=c++0x
       working-directory: array_merge
     - name: Run test
       run: valgrind -v --leak-check=full --show-leak-kinds=all --error-exitcode=1 ./array_merge_test