gdb: add declaration for _initialize_gdbarch in gdbarch.sh

gdb: add declaration for _initialize_gdbarch in gdbarch.sh

In commit

  gdb: add back declarations for _initialize functions
  6c2659886f7018fcca26ee0fc813bc9748fb8513

I wrongfully edited gdbarch.c, instead of editing gdbarch.sh and
re-generating gdbarch.c.  This patch fixes gdbarch.sh to add a
declaration for _initialize_gdbarch.  gdbarch.c is not changed, as the
output of gdbarch.sh now matches the current state of gdbarch.c.

gdb/ChangeLog:

	* gdbarch.sh: Add declaration for _initialize_gdbarch.