kbuild: distinguish between errors and warnings in modpost

Some of modpost's warnings are fatal, and some are not.  Adopt the
compiler distinction between errors and warnings by calling merror()
for fatal diagnostics and warn() for non-fatal ones.
merror() was used as replacemtn for error() to avoid clash with glibc

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>