Add extern "C" to declarations of C symbols

Add extern "C" to declarations of C symbols

These symbols are defined in C code, so in C++ mode we need to use
extern "C" to declare them.  As extern "C" can't be used inside a
function's scope, we move the declarations to the global scope at the
same time.

gdb/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* cli-out.c (_rl_erase_entire_line): Move declaration out of
	cli_mld_erase_entire_line, and make it extern "C".
	* common/common-defs.h (EXTERN_C): New.
	* completer.c (_rl_completion_prefix_display_length)
	(_rl_print_completions_horizontally, QSFUNC): Move declarations
	out of gdb_display_match_list_1.
	(_rl_qsort_string_compare): Move declaration out of
	gdb_display_match_list_1, and make it extern "C".
	* defs.h (re_comp): Use EXTERN_C.
	* maint.c (_mcleanup): Move declaration out of mcleanup_wrapper,
	and make it extern "C".
	(monstartup): Move declaration out of maintenance_set_profile_cmd,
	and make it extern "C".
	(main): Move declaration out of maintenance_set_profile_cmd.
	* nat/linux-ptrace.c (linux_ptrace_attach_fail_reason_string): Use
	EXTERN_C.