[OpenMP]Fix trivial build failure in MacOS

MacOS build of LLVM with OpenMP enabled fails with an error
that it doesn't know what std::abs is. Fix by including <cmath>
so that the relevant function declaration is included.

No functional change intended.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D150687

openmp/runtime/src/kmp_collapse.cpp

@@ -18,6 +18,8 @@
 #include "kmp_str.h"
 #include "kmp_collapse.h"
 
+#include <cmath>
+
 #if OMPT_SUPPORT
 #include "ompt-specific.h"
 #endif