Fix constant type

Remove the unnecessary cast to long for DCHECK, a nit that was omitted
from the last CL, https://android-review.googlesource.com/#/c/248243/.

Bug: b/28856653
Test: test-art-host-gtest-dex_file_test
Change-Id: I8ca18d19cbac61e40ddfbe32729aef5c20347174
diff --git a/runtime/dex_file.cc b/runtime/dex_file.cc
index dff2802..a6eb5f6 100644
--- a/runtime/dex_file.cc
+++ b/runtime/dex_file.cc
@@ -441,7 +441,7 @@
                                                    const OatDexFile* oat_dex_file,
                                                    std::string* error_msg) {
   DCHECK(base != nullptr);
-  DCHECK_NE(size, 0UL);
+  DCHECK_NE(size, 0U);
   CHECK_ALIGNED(base, 4);  // various dex file structures must be word aligned
   std::unique_ptr<DexFile> dex_file(
       new DexFile(base, size, location, location_checksum, mem_map, oat_dex_file));