When compiling with !ENABLE_FEATURE_GETOPT_LONG, busybox still tries
to include getopt.h which is not available; for example with uClibc
when !UCLIBC_HAS_GETOPT_LONG. getopt.h is only required
for the _long set of functions.
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>