package/boost: fix fiber dependencies

package/boost: fix fiber dependencies

Commit d39d8f7cee9b4634e6f490020204f63fae419e8e forgot to update fiber
dependencies resulting in the following build failure:

WARNING: unmet direct dependencies detected for BR2_PACKAGE_BOOST_FILESYSTEM
  Depends on [n]: BR2_PACKAGE_BOOST [=y] && BR2_TOOLCHAIN_SUPPORTS_ALWAYS_LOCKFREE_ATOMIC_INTS [=n]
  Selected by [y]:
  - BR2_PACKAGE_BOOST_FIBER [=y] && BR2_PACKAGE_BOOST [=y] && BR2_PACKAGE_BOOST_CONTEXT_ARCH_SUPPORTS [=y] && BR2_TOOLCHAIN_HAS_THREADS_NPTL [=y] && !BR2_MIPS_CPU_MIPS32 [=n] && !BR2_MIPS_CPU_MIPS64 [=n] && (BR2_TOOLCHAIN_SUPPORTS_ALWAYS_LOCKFREE_ATOMIC_INTS [=n] || BR2_TOOLCHAIN_GCC_AT_LEAST_6 [=y]) && !BR2_TOOLCHAIN_HAS_GCC_BUG_64735 [=n]

Fixes:
 - http://autobuild.buildroot.org/results/40b8af6ce273ebd5fca56d6734f64dacd562efb1

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Michael Nosthoff <buildroot@heine.tech>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>