[xml] minor nuisance in xmlIO.h/valid.h

[robert <robert xml 00008 org>]

I've stumbled upon a minor nuisance in xmlIO.h and valid.h: when compiling
my project with rather pedantic flags, gcc complaints about the prototypes
of xmlRegisterInputCallbacks() and others.

The reason for this is that the prototypes contain names which shadow
global declarations:
../xslt/libs/include/libxml/valid.h:108: warning: declaration of `glob'
 shadows global declaration

This occurs in the prototypes of:
xmlRegisterInputCallbacks() (xmlIO.h: match, open, read, close)
xmlRegisterOutputCallbacks() (xmlIO.h: match, open, read, close)
xmlSprintfElementContent() (valid.h: glob)

Removing the declarations within the prototypes fixes the problem for me.

                                                                    robert