Fix GN UBSanVPtr build of LLVM 10 JIT

https://ci.chromium.org/p/chromium/builders/ci/UBSan%20vptr%20Release

ERROR at //third_party/swiftshader/third_party/llvm-10.0/BUILD.gn:132:5: Undefined identifier.
    sources += [ "llvm/lib/Target/ARM/MCTargetDesc/ARMTargetStreamer.cpp" ]
    ^------

Bug: b/152339534
Change-Id: I59ae43c44ed22b95c836188897497723adcb9a23
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46668
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/third_party/llvm-10.0/BUILD.gn b/third_party/llvm-10.0/BUILD.gn
index bd69965..583b7dd 100644
--- a/third_party/llvm-10.0/BUILD.gn
+++ b/third_party/llvm-10.0/BUILD.gn
@@ -129,7 +129,7 @@
   # builds require that all declared classes have a definition even if they're
   # not used.
   if (is_ubsan_vptr) {
-    sources += [ "llvm/lib/Target/ARM/MCTargetDesc/ARMTargetStreamer.cpp" ]
+    sources = [ "llvm/lib/Target/ARM/MCTargetDesc/ARMTargetStreamer.cpp" ]
   }
 
   deps = [