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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@19059 91177308-0d34-0410-b5e6-96231b3b80d8

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;