preproc: error out if an include file exists but can't be opened

If an include file exists, but cannot be opened, that is still a
critical error.

However, downgrade this from a fatal to a nonfatal error. There really
isn't any reason to stop cold here.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

include/compiler.h

@@ -88,6 +88,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <limits.h>
+#include <errno.h>
 
 #ifdef HAVE_STRINGS_H
 # include <strings.h>