kernel: make sure that _thread_entry() declaration matches with definition

Fixes sparse warning:
  CHECK   <snip>/zephyr/kernel/thread.c
<snip>/zephyr/kernel/thread.c:184:20: error: symbol '_thread_entry' redeclared with different type (originally declared at <snip>/zephyr/kernel/include/nano_internal.h:43) - different modifiers
  CC      kernel/thread.o

Change-Id: I2223493cdf97c811c661773f8fd430e6c00cbaa0
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>