gdb: move have_ptrace_getregset declaration into gdb/nat directory

In a later commit I want to access have_ptrace_getregset from a .c
file in the nat/ directory.  To achieve this I need access to the
declaration of have_ptrace_getregset.

Currently have_ptrace_getregset is declared (and defined) twice, once
in GDB and once in gdbserver.

This commit moves the declaration into nat/linux-nat.h, but leaves the
two definitions where they are.  Now, in my later commit, I can pull
in the declaration from nat/linux-nat.h.

There should be no user visible changes after this commit.

Approved-By: Felix Willgerodt <felix.willgerodt@intel.com>