Verify that ndk_headers headers are valid C.

https://github.com/android/ndk/issues/1920 recently reported a handful
of NDK headers where C-incompatible syntax had slipped in which wasn't
caught by review. This is also something that we often manually catch
in API review, but the manual inspection is, as can be seen from that
bug, error prone.

This check is pretty stupid. I've tried on other occasions to do a
more thorough check that would build each header using the same flags
as cc_library or similar would, but was never able to get that
working. This isn't as thorough, but that's maybe not such a bad
thing, since it saves on build time to only check on variant. In
practice the rudimentary approach I've taken here would have caught
every instance reported in the bug above, and probably all the other
cases that we'd previously caught in API review as well, so it's
definitely a step in the right direction if not good enough forever.

Bug: http://b/113359184
Test: m ndk
Change-Id: Id4e8cc511413acc61c4f625f25c3004d7439263c