Make built-in TLS c89-friendly

[cpq]

C89 nits in tls_builtin.c

Testing method:

1. Install wine
2. Copy VC98 from docker image mdashnet/vc98:/vc to ~/vc98
3. Set up 3 env variables: Path=$HOME/vc98/bin, Lib=$HOME/vc98/lib, Include=$HOME/vc98/include
4. Build and test the server:

$ cd tutorials/http/http-server
$ make -C ../../../test/ mongoose.{c,h} && make CC="wine cl" CFLAGS=/MD OUT=/Feprog.exe && wine prog.exe -l https://0.0.0.0:8443

[scaprile]

Testing method:

Run the unit test ?

mongoose/.github/workflows/nightly.yml

Lines 184 to 211 in 715f98c

	windows:
	runs-on: ubuntu-latest
	strategy:
	fail-fast: false
	max-parallel: 1 # no parallel runs, to minimize MQTT errors
	matrix:
	target: [vc98, vc17, vc22, mingw, mingw++]
	ssl: ["", BUILTIN]
	select: [-DMG_ENABLE_POLL=0, -DMG_ENABLE_POLL=1]
	exclude:
	- target: vc98
	select: -DMG_ENABLE_POLL=1
	name: windows ${{ matrix.target }} SSL=${{ matrix.ssl }} TFLAGS=${{ matrix.select }}
	env:
	SSL: ${{ matrix.ssl }}
	TFLAGS: ${{ matrix.select }} -DMQTT_LOCALHOST -DNO_ABORT
	steps:
	- uses: actions/checkout@v4
	with: { fetch-depth: 2 }
	- uses: webfactory/ssh-agent@v0.9.1
	with:
	ssh-private-key: ${{ secrets.HEALTH_TESTS_SSH_KEY }}
	- run: ./test/setup_mqtt_server.sh && make -C test ${{ matrix.target }} > log
	- if: success() || failure()
	run: |
	cat log
	test/health.awk < log > json
	scp -o "StrictHostKeyChecking=no" json "root@176.9.217.245:/data/downloads/health/windows_${{ matrix.target }}_${{ matrix.cc }}_${{ matrix.ssl }}_${{ matrix.select }}_$(date +"%Y%m%d").json"

The whole test suit runs at night, can be run manually, or that can be copied to on_demand.yml and be run on GH, or just run make -C test vc98 SSL=BUILTIN and our unit tests are run locally on the Docker image.

Is there anything special with http_server itself ? The unit tests should be a more comprehensive test

Have manually run this test on this branch in my machine, and tests now pass. Left it running in GH here: https://github.com/cesanta/mongoose/actions/runs/21455806896/job/61796222093
This is one of a series of sloooooooooooooow tests that was moved to nightly/manual runs, so before sending a PR one can run a manual test or, if merged, check the next day for unwanted side effects.