Move C_VISIBILITY_PRESET back to src/CMakeLists.txt.

This reverts a change by @compnerd in commit
b6ffaca93e2b539ec407aeb4fd588c7f9441e7a9.
We don't want this for api_tests, as it triggers this warning:

```
CMake Warning (dev) at api_test/CMakeLists.txt:1 (add_executable):
  Policy CMP0063 is not set: Honor visibility properties for all target
  types.  Run "cmake --help-policy CMP0063" for policy details.  Use the
  cmake_policy command to set the policy and suppress this warning.

  Target "api_test" of type "EXECUTABLE" has the following visibility
  properties set for C:

    C_VISIBILITY_PRESET

  For compatibility CMake is not honoring them for this target.
This warning is for project developers.  Use -Wno-dev to suppress it.
```

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