Work around a Clang modules build issue.

See:
https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/40636/consoleFull#-39956214149ba4694-19c4-4d7e-bec5-911270d8a58c

```
llvm/lib/Support/Valgrind.cpp:37:63: error: missing '#include <stddef.h>'; 'size_t' must be declared before it is used
void llvm::sys::ValgrindDiscardTranslations(const void *Addr, size_t Len) {
                                                              ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/13.0.0/include/stddef.h:46:23: note: declaration here is not visible
typedef __SIZE_TYPE__ size_t;
                      ^
1 error generated.
```

rdar://88049280