package/llvm: add dependency on wchar

Extract from log:

output/build/llvm-5.0.1/include/llvm/Support/ConvertUTF.h:203:53:
error: ‘std::wstring’ has not been declared
bool ConvertUTF8toWide(llvm::StringRef Source, std::wstring &Result);

Also propagate dependency to mesa3d

Signed-off-by: Valentin Korenblit <valentin.korenblit@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>