Fix werror builds with clang

commit f65015d9

validate: fix -Werror=unused-function with clang

[3623/4053] Compiling C object 'subprojects/gst-devtools/validate/plugins/flow/697521d@@gstvalidateflow@sha/gstvalidateflow.c.o'.
../subprojects/gst-devtools/validate/plugins/flow/gstvalidateflow.c:85:1: warning: unused function 'VALIDATE_IS_FLOW_OVERRIDE' [-Wunused-function]
G_DECLARE_FINAL_TYPE (ValidateFlowOverride, validate_flow_override,
^
/usr/include/glib-2.0/gobject/gtype.h:1407:26: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
  static inline gboolean MODULE##_IS_##OBJ_NAME (gpointer ptr) {                                         \
                         ^
<scratch space>:129:1: note: expanded from here
VALIDATE_IS_FLOW_OVERRIDE
^

commit 7409a603

validate: fix -Werror=format-nonliteral build with clang

[3470/4053] Compiling C object 'subprojects/gst-devtools/validate/gst/validate/28db7b6@@gstvalidatetracer@sha/gst-validate-reporter.c.o'.
../subprojects/gst-devtools/validate/gst/validate/gst-validate-reporter.c:186:31: warning: format string is not a string literal [-Wformat-nonliteral]
  message = g_strdup_vprintf (format, vacopy);
                              ^~~~~~
[3487/4053] Compiling C object 'subprojects/gst-devtools/validate/gst/validate/28db7b6@@gstvalidatetracer@sha/gst-validate-report.c.o'.
../subprojects/gst-devtools/validate/gst/validate/gst-validate-report.c:1007:34: warning: format string is not a string literal [-Wformat-nonliteral]
  tmp = gst_info_strdup_vprintf (format, args);
                                 ^~~~~~
[76/151] Compiling C object 'subprojects/gst-devtools/validate/plugins/flow/697521d@@gstvalidateflow@sha/gstvalidateflow.c.o'.
../subprojects/gst-devtools/validate/plugins/flow/gstvalidateflow.c:125:65: warning: format string is not a string literal [-Wformat-nonliteral]
  if (!flow->error_writing_file && vfprintf (flow->output_file, format, ap) < 0) {
                                                                ^~~~~~