the github action runner seems to need libfl-dev in addition to flex

.github/workflows/dos-djgpp.yml

@@ -40,7 +40,7 @@ jobs:
       - name: Setup build environment
         run: |
           sudo apt -y update
-          sudo apt -y install build-essential cmake wget 7zip git flex nasm libslang2-dev pkg-config libslang2-modules gcc-multilib
+          sudo apt -y install build-essential cmake wget 7zip git flex libfl-dev nasm libslang2-dev pkg-config libslang2-modules gcc-multilib
 
       - name: Download and Setup DJGPP Toolchain
         run: |