Remove the EXTERN define from stabsread.h

Remove the EXTERN define from stabsread.h

This removes the EXTERN define from stabsread.h.  This is the only
spot that still uses this approach, and it interfered with sorting the
includes in stabsread.c.

Tested by rebuilding.

gdb/ChangeLog
2019-01-22  Tom Tromey  <tom@tromey.com>

	* stabsread.c (EXTERN): Do not define.
	(symnum, next_symbol_text_func, processing_gcc_compilation)
	(within_function, global_sym_chain, global_stabs)
	(previous_stab_code, this_object_header_files)
	(n_this_object_header_files)
	(n_allocated_this_object_header_files): Define.
	* stabsread.h (EXTERN): Never define.  Use "extern".