Fix missing ctype.h includes

libavutil/common.h stopped including ctype.h, and some source files were
relying on recursive inclusion of this header.

sub/subassconvert.c

@@ -24,6 +24,7 @@
 #include <stdio.h>
 #include <stdarg.h>
 #include <stdbool.h>
+#include <ctype.h>
 
 #include "core/mp_msg.h"
 #include "subassconvert.h"

sub/subreader.c

@@ -26,6 +26,7 @@
 #include <string.h>
 #include <sys/types.h>
 #include <dirent.h>
+#include <ctype.h>
 
 #include "config.h"
 #include "core/mp_msg.h"