Move declaration of vtbl_ptr_name to the header.

Move declaration of vtbl_ptr_name to the header.

There are conflicting comments about whether this was
introduced in GCC 2.4.5 or GCC 2.6 and I don't know
which one is correct...

gdb/ChangeLog:

2019-10-07  Christian Biesinger  <cbiesinger@google.com>

	* c-lang.h (vtbl_ptr_name): Declare.
	* cp-valprint.c (vtbl_ptr_name): Remove "extern" now that we get
	it from the header.
	* stabsread.c (define_symbol): Remove declaration of vtbl_ptr_name.