Enable -Wmissing-declarations diagnostic

Enable -Wmissing-declarations diagnostic

Now that most warnings of this kind are fixed, we can enable
-Wmissing-declarations.  I say "most", because it is likely that there
are some more in some configurations I am not able to build, but they
should be pretty easy to fix.

gdb/ChangeLog:

	* warning.m4: Add -Wmissing-declarations to build_warnings.
	* configure: Re-generate.

gdb/gdbserver/ChangeLog:

	* configure: Re-generate.

Change-Id: Iae9b59f22eb5dd1965d09f34c5c9e212cddf67ba