src: guard for double declaration of curl_ca_embed in unity builds

[vszakats]

Seen with curl-for-win linux-musl-from-mac build with gcc 9.2.0.

n file included from /Users/runner/work/curl-for-win/curl-for-win/curl/_x64-linux-musl-bld/src/CMakeFiles/curl.dir/Unity/unity_0_c.c:136:
/Users/runner/work/curl-for-win/curl-for-win/curl/_x64-linux-musl-bld/src/tool_ca_embed.c:4:28: warning: redundant redeclaration of 'curl_ca_embed' [-Wredundant-decls]
    4 | extern const unsigned char curl_ca_embed[];
      |                            ^~~~~~~~~~~~~
In file included from /Users/runner/work/curl-for-win/curl-for-win/curl/_x64-linux-musl-bld/src/CMakeFiles/curl.dir/Unity/unity_0_c.c:88:
/Users/runner/work/curl-for-win/curl-for-win/curl/src/tool_operate.c:107:28: note: previous declaration of 'curl_ca_embed' was here
  107 | extern const unsigned char curl_ca_embed[];
      |                            ^~~~~~~~~~~~~

https://github.com/curl/curl-for-win/actions/runs/11192203640/job/31116070669#step:3:4894

Follow-up to 8a3740b #14059

[dfandrich]

Analysis of PR #15307 at e2add64b:

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-0--2] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-0--1] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-0-0] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-1--2] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-1--1] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-1-0] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-2--2] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

There are more failures, but that's enough from Gha.

Generated by Testclutch

[vszakats]

Fix confirmed working here:
https://github.com/curl/curl-for-win/actions/runs/11367258574/job/31619571137