ANDROID: ftrace: add trace_seq declare to fix build error

commit 1a094aba37 ("Revert "ANDROID: ftrace: vendor hook for
ftrace dump on oops"") remove all define, but trace_seq was not
added back, it was still needed by android_vh_ftrace_dump_buffer,
add it here.

Bug: 177387994
Change-Id: Ifab789dc09084a037db3cd0df1f3fd0cf5b2fa9c
Signed-off-by: Zhou Jian <quic_jianzhou@quicinc.com>

include/trace/hooks/ftrace_dump.h

@@ -9,6 +9,8 @@
 
 #include <trace/hooks/vendor_hooks.h>
 
+struct trace_seq;
+
 DECLARE_HOOK(android_vh_ftrace_oops_enter,
 	TP_PROTO(bool *ftrace_check),
 	TP_ARGS(ftrace_check));