core.platform.api.stubs: Fix OpenJDK9 compilation.

Commit 3513bab81e8d4462204b0936e185248a014d23ce removed the
--patch-module=java.base=. parameter for the javac invocation
but it is still needed because the (stub) sources for this
make target include classes from java.lang and friends.

This CL adds it back.

Test: Checked that the following fails before but passes after this CL:
      EXPERIMENTAL_USE_OPENJDK9=true make core.platform.api.stubs
Fixes: 117816675

Change-Id: I2495fa9c4f88f477dd21dcb9736b6176a892276a
1 file changed