um: Correctly check for PTRACE_GETRESET/SETREGSET

When checking for PTRACE_GETRESET/SETREGSET, make sure that
the correct header file is included. We need linux/ptrace.h
which contains all ptrace UAPI related defines.
Otherwise #if defined(PTRACE_GETRESET) is always false.

Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>