gdb: Add -Wno-mismatched-tags

gdb: Add -Wno-mismatched-tags

clang complains that for some types, we use both the class and struct
keywords in different places.  It's not really a problem, so I think we
can safely turn this warning off.

gdb/ChangeLog:

	* configure: Re-generate.
	* warning.m4 (build_warnings): Add -Wno-mismatched-tags.

gdb/gdbserver/ChangeLog:

	* configure: Re-generate.