[libc] fix more -Wmissing-brace (#77382)

Similar to #77345, the buildbots are observing similar warnings for the
sse2
implementation.

llvm-project/libc/src/__support/HashTable/sse2/bitmask_impl.inc:36:13:
    error: suggest braces around initialization of subobject
    [-Werror,-Wmissing-braces]
    return {bitmask};
            ^~~~~~~
            {      }
llvm-project/libc/src/__support/HashTable/sse2/bitmask_impl.inc:45:13:
    error: suggest braces around initialization of subobject
    [-Werror,-Wmissing-braces]
    return {static_cast<uint16_t>(~mask_available().word)};
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            {                                            }

Link:
https://lab.llvm.org/buildbot/#/builders/163/builds/49350/steps/8/logs/stdio
Link: https://github.com/llvm/llvm-project/pull/74506