kbuild/modpost: Use warn() for announcing section mismatches

modpost: Use warn() for announcing section mismatches, for easy grepping for
warnings in build logs.

Also change an existing call from fprintf() to warn() while we're at it.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>