[lldb][Utility] Fix GCC suggest braces warning for checksum value

[4314/5686] Building CXX object tools/lldb/sou...lity/CMakeFiles/lldbUtility.dir/Checksum.cpp.o
<...>/Checksum.cpp:43:46: warning: suggest braces around initialization of subobject [-Wmissing-braces]
llvm::MD5::MD5Result Checksum::g_sentinel = {0, 0, 0, 0, 0, 0, 0, 0,
                                             ^~~~~~~~~~~~~~~~~~~~~~~
                                             {