MIBenum values are from an IANA registry, not a WAP specification; add
<epan/iana_charsets.h> to declare the MIBenum -> Wireshark encoding
mapper routine and the value_string_ext for MIBenum values, and
epan/iana_charsets.c to define them.
Change-Id: I6d9c82cd011bd5211c688322e6423de38e161f41
Reviewed-on: https://code.wireshark.org/review/15298
Reviewed-by: Guy Harris <guy@alum.mit.edu>