ci: install gperf in CI workflow

The build requires gperf to generate src/json_tokens.cpp from the
.gperf source. The CI jobs were failing at CMake configure time with:

  CMake Error at CMakeLists.txt:108 (find_program):
    Could not find GPERF_EXECUTABLE using the following names: gperf

Add gperf to the apt-get install step so all build dependencies are
present on the runners.

.gitea/workflows/ci.yml

@@ -46,7 +46,7 @@ jobs:
       - name: Install deps
         run: |
           sudo apt-get update
-          sudo apt-get install -y build-essential clang cmake
+          sudo apt-get install -y build-essential clang cmake gperf
 
       - name: Build and install weaseljson
         run: |