Fix missing-prototypes warnings in gnu-nat.c

Fix missing-prototypes warnings in gnu-nat.c

A recent change (commit 3398af6aa352b0611bc9d66aed72080a876e42d4)
in gnu-nat.c causes the some missing-prototypes warnings,

../../../git/gdb/gnu-nat.c:1864:1: error: no previous prototype for 'S_proc_pid2task_reply' [-Werror=missing-prototypes]
../../../git/gdb/gnu-nat.c:1866:1: error: no previous prototype for 'S_proc_task2pid_reply' [-Werror=missing-prototypes]
../../../git/gdb/gnu-nat.c:1868:1: error: no previous prototype for 'S_proc_task2proc_reply' [-Werror=missing-prototypes]

A new macro ILL_RPC was added recently, which defines some external
functions.  However, they are not declared and GCC complains about this.

This patch is to add the declarations of these external function in
macro ILL_RPC.

gdb:

2014-02-17  Yao Qi  <yao@codesourcery.com>

	* gnu-nat.c (ILL_RPC): Declare defined function.