Fix path to v8 include files

I believe the paths to the V8 include headers are incorrect. The paths
to other sources seem to be relative to the parent directory.

When building Node.js I get the following warning on Windows:
Warning: Missing input files:
deps\v8\src\..\..\include\v8-inspector-protocol.h
deps\v8\src\..\..\include\v8-inspector.h

This commit updates the two include paths.

Bug: 
Change-Id: I51a057abba61e294e7811ba69db03e283b0bdc3f
Reviewed-on: https://chromium-review.googlesource.com/743981
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49121}

src/inspector/inspector.gypi

@@ -31,8 +31,8 @@
     'inspector_all_sources': [
       '<@(inspector_generated_sources)',
       '<(inspector_generated_injected_script)',
-      '../../include/v8-inspector.h',
-      '../../include/v8-inspector-protocol.h',
+      '../include/v8-inspector.h',
+      '../include/v8-inspector-protocol.h',
       'inspector/injected-script.cc',
       'inspector/injected-script.h',
       'inspector/inspected-context.cc',