fix: add missing exports

[marclaval]

When building ng-bootstrap with Ivy, I'm getting the following error:

BUILD ERROR
src/buttons/label.ts(8,14): error TS-993001: Unsupported private class NgbButtonLabel. This class is visible to consumers via NgbButtonsModule -> NgbButtonLabel, but is not exported from the top-level library entrypoint.
src/buttons/radio.ts(80,14): error TS-993001: Unsupported private class NgbRadio. This class is visible to consumers via NgbButtonsModule -> NgbRadio, but is not exported from the top-level library entrypoint.
src/dropdown/dropdown.ts(71,14): error TS-993001: Unsupported private class NgbDropdownAnchor. This class is visible to consumers via NgbDropdownModule -> NgbDropdownAnchor, but is not exported from the top-level library entrypoint.
src/dropdown/dropdown.ts(95,14): error TS-993001: Unsupported private class NgbDropdownToggle. This class is visible to consumers via NgbDropdownModule -> NgbDropdownToggle, but is not exported from the top-level library entrypoint.
src/dropdown/dropdown.ts(28,14): error TS-993001: Unsupported private class NgbDropdownMenu. This class is visible to consumers via NgbDropdownModule -> NgbDropdownMenu, but is not exported from the top-level library entrypoint.

I don't know why it happens ... Adding the missing exports solves the issue.

[codecov-io]

Codecov Report

Merging #2997 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2997   +/-   ##
=======================================
  Coverage   92.26%   92.26%           
=======================================
  Files          91       91           
  Lines        2870     2870           
  Branches      463      463           
=======================================
  Hits         2648     2648           
  Misses        163      163           
  Partials       59       59

Flag	Coverage Δ
#e2e	60.1% <ø> (ø)	⬆️
#unit	90.64% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
src/index.ts	100% <ø> (ø)	⬆️
src/dropdown/dropdown.module.ts	100% <ø> (ø)	⬆️

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[maxokorokov]

LGTM, thanks!