ssaparse: include required system headers for isspace() and sscanf() functions

Newer compilers ( clang 15 ) have turned stricter and errors out instead
of warning on implicit function declations

Fixes
gstssaparse.c:297:12: error: call to undeclared library function 'isspace' with type 'int (int)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    while (isspace(*t))

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2882>

subprojects/gst-plugins-base/gst/subparse/gstssaparse.c

@@ -24,6 +24,8 @@
 #include "config.h"
 #endif
 
+#include <ctype.h>              /* isspace() */
+#include <stdio.h>              /* sscanf() */
 #include <stdlib.h>             /* atoi() */
 #include <string.h>