Fix missing headers for vndk snapshotted libc

VNDK snapshot depends on addExportedGeneratedHeaders to grab generated
headers. This change adds a missing addExportedGeneratedHeaders call to
make libc's headers captured correctly.

Bug: 181326838
Test: VNDK_SNAPSHOT_BUILD_ARTIFACTS=true m dist vndk && unzip -l
out/dist/*.zip

Change-Id: Iaa89b5255c836761a26c77bb27f35f768f49039e