Use Config/alloca.h instead of alloca.h - pointed out by Duraid and Misha

llvm-svn: 19059
diff --git a/llvm/tools/llvmc/CompilerDriver.cpp b/llvm/tools/llvmc/CompilerDriver.cpp
index 6578815..6846fbc 100644
--- a/llvm/tools/llvmc/CompilerDriver.cpp
+++ b/llvm/tools/llvmc/CompilerDriver.cpp
@@ -21,7 +21,7 @@
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include <iostream>
-#include <alloca.h>
+#include "llvm/Config/alloca.h"
 
 using namespace llvm;