ubi-utils: fix warning in fprintf() code

ubi-utils: fix warning in fprintf() code

When building with gcc security warnings enabled, the ubimirror.c code
triggers this warning:
./src/ubimirror.c: In function 'main':
./src/ubimirror.c:206: error: format not a string literal and no format arguments

Since the buffer in question should be a straight string anyways, avoid
ugly printf exploits by outputting the string indirectly:
	printf("%s", buffer)

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>