math: actually pull the declarations/overloads into std

The previous changes missed the change to include/cmath.  These changes
allow some of the rand.distribution tests to pass on Windows.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294957 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed