[OpenMP] Move header inclusion out of 'extern "C"'

This leads to problems when compiling C++ code with libc++ for Nvidia GPUs
because Clang now uses wrappers for math functions that might include
C++ templates not allowed in 'extern "C"'.

Differentiel Revision: https://reviews.llvm.org/D64625

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@366229 91177308-0d34-0410-b5e6-96231b3b80d8