installer: don't install node_internals.h

As the name suggests, it's for internal use only, so don't install it.

Including it in an add-on doesn't work because the file depends on other
header files that are not installed.

Adding it to the install list appears to have been an oversight in
commit 32478acf ("build: unix install node and dep library headers").

PR-URL: https://github.com/nodejs/node/pull/6913


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>