Nearly every header in the boost codebase is UTF-8, but here there
are a few headers which are using Windows-1252, which makes it impossible
for some tools to parse those files. This patch just reencodes them
with UTF-8 like the rest of the codebase. I checked that the name of the
author is still correct after this change and the reencoded string
literal was the plus-minus-sign.
No functional change intended.
