[Support] Add missing <cstdint> header to Base64.h

Without the change llvm build fails on this week's gcc-13 snapshot as:

    [ 91%] Building CXX object unittests/Support/CMakeFiles/SupportTests.dir/Base64Test.cpp.o
    In file included from llvm/unittests/Support/Base64Test.cpp:14:
    llvm/include/llvm/Support/Base64.h: In function 'std::string llvm::encodeBase64(const InputBytes&)':
    llvm/include/llvm/Support/Base64.h:29:5: error: 'uint32_t' was not declared in this scope
       29 |     uint32_t x = ((unsigned char)Bytes[i] << 16) |
          |     ^~~~~~~~