Fix a mkdef.pl warning

mkdef.pl was issuing the following error:

 Use of uninitialized value within %tag in numeric eq (==) at
 util/mkdef.pl line 560, <IN> line 92

This was because it was treating a __cplusplus "#ifdef" check as a "tag"
but then skipping over the corresponding "#endif". Therefore after
processing a file it still had "left over" tags from processing the
previous file. It was also getting confused by "#if" checks that didn't
match is pre-defined styles.

Reviewed-by: Richard Levitte <levitte@openssl.org>