Make GPU support optional to enable RISC-V builds without libdrm

- Add 'gpu' feature flag (enabled by default)
- Make gfxinfo dependency optional
- Provide no-op GPU metrics when gpu feature disabled
- Disable GPU support for RISC-V builds in CI (libdrm unavailable)
- All other architectures (amd64, arm64, armhf) still get GPU support

.github/workflows/build-deb.yml

@@ -115,10 +115,16 @@ jobs:
         run: |
           cargo deb --package socktop --target ${{ matrix.target }} --no-strip
 
-      - name: Build socktop_agent .deb package
+      - name: Build socktop_agent .deb package (with GPU support)
+        if: matrix.target != 'riscv64gc-unknown-linux-gnu'
         run: |
           cargo deb --package socktop_agent --target ${{ matrix.target }} --no-strip
 
+      - name: Build socktop_agent .deb package (without GPU support for RISC-V)
+        if: matrix.target == 'riscv64gc-unknown-linux-gnu'
+        run: |
+          cargo deb --package socktop_agent --target ${{ matrix.target }} --no-strip --no-default-features
+
       - name: Copy packages to debs directory
         run: |
           mkdir -p debs