[Xray] fix c99 warning build about flexible array semantics

Reviewers: dberris

Reviewed By: dberris

Differential Revision: https://reviews.llvm.org/D49590

llvm-svn: 337536

compiler-rt/lib/xray/xray_segmented_array.h

@@ -40,7 +40,7 @@ template <class T> class Array {
   // We want each segment of the array to be cache-line aligned, and elements of
   // the array be offset from the beginning of the segment.
   struct Segment : SegmentBase {
-    char Data[];
+    char Data[1];
   };
 
 public: