[Native][MIPS] Add stddef.h to platform.posix on linux

For some reason, ptrdiff_t is missing in platform.posix on MIPS,
yet referenced from platform.linux.__morecore. Fixed by adding parent
header.
This fix is ABI for MIPS targets breaking because some declarations move
from platform.linux to platform.posix package.

^KT-46957 Fixed

kotlin-native/platformLibs/src/platform/linux/posix.def

@@ -3,7 +3,7 @@ headers = alloca.h ar.h assert.h complex.h ctype.h dirent.h dlfcn.h err.h errno.
     fenv.h fnmatch.h fts.h ftw.h getopt.h grp.h inttypes.h libgen.h limits.h \
     locale.h math.h memory.h netdb.h paths.h poll.h \
     pthread.h pwd.h regex.h resolv.h sched.h search.h semaphore.h setjmp.h sgtty.h signal.h \
-    stdatomic.h stdint.h stdio.h stdlib.h string.h strings.h syslog.h termios.h \
+    stdatomic.h stdint.h stddef.h stdio.h stdlib.h string.h strings.h syslog.h termios.h \
     time.h ucontext.h unistd.h utime.h utmp.h wchar.h wctype.h xlocale.h \
     net/ethernet.h net/if.h net/if_arp.h net/route.h \
     netinet/icmp6.h netinet/if_ether.h netinet/in.h netinet/in_systm.h \