perf synthetic-events: Fix uninitialized 'kernel_thread' variable

perf build fails on 5.12.0rc2 on s390 with this error message:

util/synthetic-events.c: In function
				‘__event__synthesize_thread.part.0.isra’:
util/synthetic-events.c:787:19: error: ‘kernel_thread’ may be
    used uninitialized in this function [-Werror=maybe-uninitialized]
    787 |   if (_pid == pid && !kernel_thread) {
        |       ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~

The build succeeds using command 'make DEBUG=y'.

The variable kernel_thread is set by this function sequence:

__event__synthesize_thread()
|    defines bool kernel_thread; as local variable and calls
+--> perf_event__prepare_comm(..., &kernel_thread)
     +--> perf_event__get_comm_ids(..., bool *kernel);
          On return of this function variable kernel is always
          set to true or false.

To prevent this compile error, assign variable kernel_thread
a value when it is defined.

Output after:

  [root@m35lp76 perf]# make  util/synthetic-events.o
  ....
   CC       util/synthetic-events.o
  [root@m35lp76 perf]#

Fixes: c1b90795 ("perf tools: Skip PERF_RECORD_MMAP event synthesis for kernel threads")
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Link: http://lore.kernel.org/lkml/20210309110447.834292-1-tmricht@linux.ibm.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>