[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) |
| ^~~~~~~~