build: fix typo in non-essential source file name

Depend on src/inspector_agent.h, src/inspector-agent.h does not exist.

The typo didn't break the build but it made some of the gyp-based
tooling complain.

PR-URL: https://github.com/nodejs/node/pull/7945
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

node.gyp

@@ -319,7 +319,7 @@
             'src/inspector_agent.cc',
             'src/inspector_socket.cc',
             'src/inspector_socket.h',
-            'src/inspector-agent.h',
+            'src/inspector_agent.h',
           ],
           'dependencies': [
             'deps/v8_inspector/platform/v8_inspector/v8_inspector.gyp:v8_inspector_stl',