linuxptp: add patch to fix no-thread/linuxthreads uClibc builds

linuxptp: add patch to fix no-thread/linuxthreads uClibc builds

linuxptp missing.h header implements a replacement clock_nanosleep()
function, which was used when the thread implementation was not NPTL,
because uClibc failed to provide clock_nanosleep() in such
configurations.

However, uClibc-ng has fixed this problem upstream, and has backported
this change in Buildroot in patch
package/uclibc/0002-librt-declare-clock_nanosleep-independent-of-thread-.patch
(the code is upstream and will be part of uClibc-ng 1.0.31). Due to
this, there is now a conflicting definition of clock_nanosleep()
between the C library and the linuxptp missing.h code, which manifests
itself by the following build failure:

missing.h:117:19: error: static declaration of 'clock_nanosleep' follows non-static declaration
 static inline int clock_nanosleep(clockid_t clock_id, int flags,
                   ^~~~~~~~~~~~~~~
In file included from clockadj.h:24:0,
                 from clockadj.c:24:

This commit fixes that by adding a patch that removes the replacement
clock_nanosleep() implementation from the linuxptp code base.

Fixes:

  http://autobuild.buildroot.net/results/bf400095a853f5beb28c77a50fcffefe52c3d769/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>