xtensa: Add missing statics

xtensa: Add missing statics

This actually fixes the build in C:

/home/simark/src/binutils-gdb/gdb/xtensa-linux-nat.c:100:1: error: no previous prototype for ‘supply_gregset_reg’ [-Werror=missing-prototypes]
 supply_gregset_reg (struct regcache *regcache,
 ^
/home/simark/src/binutils-gdb/gdb/xtensa-linux-nat.c:257:1: error: no previous prototype for ‘xtensa_linux_fetch_inferior_registers’ [-Werror=missing-prototypes]
 xtensa_linux_fetch_inferior_registers (struct target_ops *ops,
 ^
/home/simark/src/binutils-gdb/gdb/xtensa-linux-nat.c:272:1: error: no previous prototype for ‘xtensa_linux_store_inferior_registers’ [-Werror=missing-prototypes]
 xtensa_linux_store_inferior_registers (struct target_ops *ops,
 ^
cc1: all warnings being treated as errors

These functions are local to this file, so they should be static.

gdb/ChangeLog:

	* xtensa-linux-nat.c (supply_gregset_reg): Make static.
	(xtensa_linux_fetch_inferior_registers): Likewise.
	(xtensa_linux_store_inferior_registers): Likewise.