Fix class/struct mismatch.

libc++ has switched this from a class to a struct to match libstdc++.
The standard does not require either specifically, but Clang warns
about the mismatch: https://bugs.llvm.org/show_bug.cgi?id=39871.

Test: m
Bug: None
Exempt-From-Owner-Approval: Janitorial
Change-Id: Iffbd944f60eb363ef7cd8f9f2c2eea77ff967310
1 file changed