Fix PR PR16445 - gdbserver build failure on x86.

Fix PR PR16445 - gdbserver build failure on x86.

If gdb_proc_service.h ends up including linux/elf.h, we'll trip on
duplicate definitions:

 In file included from ../../../gdb/gdbserver/linux-x86-low.c:29:0:
 ../../../gdb/gdbserver/../../include/elf/common.h:36:0: error: "ELFMAG0"
                 redefined [-Werror]
 ... etc ...

Handle this the same way linux-low.c and linux-arm-low.c handle this.

gdb/gdbserver/
2014-01-17  Pedro Alves  <palves@redhat.com>

	PR PR16445
	* linux-x86-low.c (linux-x86-low.c): Don't include elf/common.h if
	ELFMAG0 is defined after including gdb_proc_service.h.