um: prctl: Do not include linux/ptrace.h

On recent toolchains we hit:
In file included from arch/x86/um/os-Linux/prctl.c:7:0:
/usr/include/linux/ptrace.h:58:8: error: redefinition of ‘struct
ptrace_peeksiginfo_args’ struct ptrace_peeksiginfo_args {
        ^
In file included from arch/x86/um/os-Linux/prctl.c:6:0:
/usr/include/sys/ptrace.h:191:8: note: originally defined here
 struct ptrace_peeksiginfo_args
        ^
make[2]: *** [arch/x86/um/os-Linux/prctl.o] Error 1
make[1]: *** [arch/x86/um/os-Linux] Error 2
make: *** [arch/x86/um] Error 2

The solution is not to include linux/ptrace.h and obtain
the arch specific ptrace command from asm/ptrace.h.
Reported-and-tested-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/x86/um/os-Linux/prctl.c

@@ -4,7 +4,7 @@
  */
 
 #include <sys/ptrace.h>
-#include <linux/ptrace.h>
+#include <asm/ptrace.h>
 
 int os_arch_prctl(int pid, int code, unsigned long *addr)
 {