Fix non-threaded Callback DESTROY #if identation

All tests successful.
Files=13, Tests=188, 19 wallclock secs ( 0.08 usr  0.09 sys +  5.32 cusr  8.78 csys = 14.27 CPU)
Result: PASS


Summary of my perl5 (revision 5 version 16 subversion 2) configuration:
   
  Platform:
    osname=cygwin, osvers=1.7.17s(0.26253), archname=cygwin-64int-debug
    uname='cygwin_nt-5.1 win 1.7.17s(0.26253) 20120801 11:02:01 i686 cygwin '
    config_args='-de -Dusedevel -Uversiononly -Dinstallman1dir=none -Dinstallman3dir=none -Dinstallsiteman1dir=none -Dinstallsiteman3dir=none -DEBUGGING -Doptimize=-g3 -Uusethreads -Dlibperl=cygperl5_16_2d-nt.dll -Dcc='gcc-4' -Dld='g++-4''
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=undef, usemultiplicity=undef
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=define, use64bitall=undef, uselongdouble=undef
    usemymalloc=y, bincompat5005=undef
  Compiler:
    cc='gcc-4', ccflags ='-DPERL_USE_SAFE_PUTENV -U__STRICT_ANSI__ -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include',
    optimize='-g3',
    cppflags='-DPERL_USE_SAFE_PUTENV -U__STRICT_ANSI__ -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.5.3', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='g++-4', ldflags =' -Wl,--enable-auto-import -Wl,--export-all-symbols -Wl,--enable-auto-image-base -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib /lib
    libs=-lgdbm -ldb -ldl -lcrypt -lgdbm_compat
    perllibs=-ldl -lcrypt
    libc=/usr/lib/libc.a, so=dll, useshrplib=true, libperl=cygperl5_16_2d-nt.dll
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=dll, d_dlsymun=undef, ccdlflags=' '
    cccdlflags=' ', lddlflags=' --shared  -Wl,--enable-auto-import -Wl,--export-all-symbols -Wl,--enable-auto-image-base -L/usr/local/lib -fstack-protector'


Characteristics of this binary (from libperl): 
  Compile-time options: DEBUGGING HAS_TIMES MYMALLOC PERLIO_LAYERS
                        PERL_DONT_CREATE_GVSV PERL_MALLOC_WRAP
                        PERL_PRESERVE_IVUV PERL_USE_DEVEL
                        PERL_USE_SAFE_PUTENV USE_64_BIT_INT USE_LARGE_FILES
                        USE_LOCALE USE_LOCALE_COLLATE USE_LOCALE_CTYPE
                        USE_LOCALE_NUMERIC USE_PERLIO USE_PERL_ATOF
  Built under cygwin
  Compiled at Nov  2 2012 11:14:46
  @INC:
    /usr/local/lib/perl5/site_perl/5.16.2/cygwin-64int-debug
    /usr/local/lib/perl5/site_perl/5.16.2
    /usr/local/lib/perl5/5.16.2/cygwin-64int-debug
    /usr/local/lib/perl5/5.16.2