Add missing variable in gyp file.

Node.js doesn't build because v8_extra_library_files is
used but not defined in v8.gyp.

We can probably clean up the build file and properly
delete v8_extra_library_files, just want to get the integration
build green again.

TBR=adamk@chromium.org

BUG=

Review-Url: https://codereview.chromium.org/2720483002
Cr-Commit-Position: refs/heads/master@{#43429}

src/v8.gyp

@@ -32,6 +32,7 @@
     'v8_vector_stores%': 0,
     'embed_script%': "",
     'warmup_script%': "",
+    'v8_extra_library_files%': [],
     'v8_experimental_extra_library_files%': [],
     'v8_enable_inspector%': 0,
     'mksnapshot_exec': '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)mksnapshot<(EXECUTABLE_SUFFIX)',