Reapply "Makefile.extrawarn: turn on missing-prototypes globally"

This reverts commit 635125d4. The issues
exposed by these flags are now resolved and the original commit can be
reapplied now. This avoids further merge conflicts with mainline.

Change-Id: I49b8126bbd3a77f8dcc711c1abbe82602b49fcb7
Signed-off-by: Carlos Llamas <cmllamas@google.com>