unwind: permit building against libstdc++

Include algorithm early as otherwise you get a number of particularly unhelpful
messages about failed static assertions.  This fixes compilation on Linux with
gcc.

llvm-svn: 237002
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 5ccb0b5f870202938dd8ad387400564cb775b9c1
diff --git a/src/libunwind.cpp b/src/libunwind.cpp
index 031ecce..8f5078b 100644
--- a/src/libunwind.cpp
+++ b/src/libunwind.cpp
@@ -19,6 +19,7 @@
 #include <tuple>
 #include <memory>
 #include <vector>
+#include <algorithm>
 
 #include "libunwind_ext.h"
 #include "config.h"