Add missing include for LLVM_VERSION_MAJOR

llvm-config.h provides LLVM_VERSION_MAJOR. The code is currently pulling
it indirectly from some other headers.

Change-Id: Iea7a24a23eb1545a76dc1aa3896da793742a29de
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/77288
Reviewed-by: Geoff Lang <geofflang@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@google.com>
Commit-Queue: Geoff Lang <geofflang@google.com>
Tested-by: Geoff Lang <geofflang@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>

src/Reactor/LLVMReactor.cpp

diff --git a/src/Reactor/LLVMReactor.cpp b/src/Reactor/LLVMReactor.cpp
index a2cd0d7..4507462 100644
--- a/src/Reactor/LLVMReactor.cpp
+++ b/src/Reactor/LLVMReactor.cpp
@@ -23,6 +23,7 @@
 #include "SIMD.hpp"
 #include "x86.hpp"
 
+#include "llvm/Config/llvm-config.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/IntrinsicsX86.h"
 #include "llvm/Support/Alignment.h"