gdbserver: set IP_AGENT_EXPORT_FUNC to static when not building IPA, add declarations

gdbserver: set IP_AGENT_EXPORT_FUNC to static when not building IPA, add declarations

Fixing the -Wmissing-declarations errors in gdbserver's tracepoint.c is
a bit tricky, because some functions are compiled for both gdbserver, in
which case they should be static, since they are only used in that file,
and for libinproctrace.so, in which case they should be externally
visible, since they need to be looked up.  In the case where they are
externally visible, -Wmissing-declarations requires that a declaration
exists (that's the point of the warning).

I've reused the IP_AGENT_EXPORT_FUNC macro to mark the functions as
static when compiled for gdbserver.  Some seemingly unnecessary
declarations are added for when compiling libinproctrace.so (thanks to
Tom for the suggestion).

gdb/gdbserver/ChangeLog:

	* tracepoint.h (IP_AGENT_EXPORT_FUNC) [!IN_PROCESS_AGENT]:
	Define to static.
	* tracepoint.c (stop_tracing, flush_trace_buffer,
	about_to_request_buffer_space, get_trace_state_variable_value,
	set_trace_state_variable_value, gdb_collect): Add declaration.

Change-Id: If9c66151bd00c3b9c5caa27a7c21c5a3a952de2a