gdb: xtensa: fix linux ptrace includes

Currently, xtensa code using the Linux ptrace interface only include
sys/ptrace.h.  This file comes from the C library (glibc and uClibc,
at least), and includes a declaration of the ptrace() functions, along
with some cross architecture constants that are mostly copied from the
file located at include/uapi/linux/ptrace.h in recent Linux kernels.

For xtensa specific constants like PTRACE_GETXTREGS and
PTRACE_SETXTREGS the asm/ptrace.h include from the Linux kernel UAPI
is needed.  The code in gdbserver xtensa specific part doesn't call
ptrace() directly, so we can remove the unneeded sys/ptrace.h include.
The gdb xtensa specific code needs both headers, since it calls
ptrace().

gdb/
	* xtensa-linux-nat.c: Include asm/ptrace.h.

gdb/gdbserver/
	* linux-xtensa-low.c: Include asm/ptrace.h instead of
	sys/ptrace.h.

gdb/xtensa-linux-nat.c

@@ -36,6 +36,7 @@
 #include <fcntl.h>
 #include <sys/procfs.h>
 #include <sys/ptrace.h>
+#include <asm/ptrace.h>
 
 #include "gregset.h"
 #include "xtensa-tdep.h"