[CMake] Fix -Wstrict-prototypes

Fixes warnings (or errors, if someone injects -Werror in their build system,
which happens in fact with some folks vendoring LLVM too) with Clang 16:
```
+/var/tmp/portage.notmp/portage/sys-devel/llvm-15.0.4/work/llvm_build-abi_x86_64.amd64/CMakeFiles/CMakeTmp/src.c:3:9: warning: a function declaration without a prototype
is deprecated in all versions of C [-Wstrict-prototypes]
-/var/tmp/portage.notmp/portage/sys-devel/llvm-14.0.4/work/llvm_build-abi_x86_64.amd64/CMakeFiles/CMakeTmp/src.c:3:9: error: a function declaration without a prototype is
deprecated in all versions of C [-Werror,-Wstrict-prototypes]
 int main() {return 0;}
         ^
          void
```

Differential Revision: https://reviews.llvm.org/D137503