Fix the return type for pager_output(..) in bootparttest and zfsboottest

pager_output(..) has always returned int. For some reason (probably the
clang 5.0 update), this mismatch now results in an error with CC=clang.

MFC after:	1 week

tools/tools/bootparttest/stub.c

@@ -44,9 +44,9 @@ Free(void *ptr, const char *file, int line)
 	return (free(ptr));
 }
 
-void
+int
 pager_output(const char *s)
 {
 
-	printf("%s", s);
+	return (printf("%s", s));
 }

tools/tools/zfsboottest/zfsboottest.c

@@ -43,11 +43,11 @@
 
 #define NBBY 8
 
-void
+int
 pager_output(const char *line)
 {
 
-	fprintf(stderr, "%s", line);
+	return (fprintf(stderr, "%s", line));
 }
 
 #define ZFS_TEST