[klibc] include: [stdio.h] add clearerr() prototype

Seen on kmod compiling:
/usr/lib/klibc/include/stdio.h:130:22: warning: no previous prototype for 'clearerr' [-Wmissing-prototypes]

Signed-off-by: maximilian attems <max@stro.at>
diff --git a/usr/include/stdio.h b/usr/include/stdio.h
index f24ab69..21243cc 100644
--- a/usr/include/stdio.h
+++ b/usr/include/stdio.h
@@ -95,6 +95,7 @@
 __extern int ferror(FILE * );
 __extern int feof(FILE *);
 __extern int fileno(FILE *);
+__extern void clearerr(FILE *);
 
 #ifndef __NO_STDIO_INLINES
 __extern_inline size_t