Add posix_memalign to API16's libc.so, and to stdlib.h for API 16, 17, 18, 19

posix_memalign is added to libc.so for API 17, 18, 19 but prototype
is missing from stdlib.h

Turns out API 16 supports posix_memalign too.  Add it as well

See b.android.com/77861

Change-Id: I940cdc60af9f98490001d67bef32fee556a3ac51
Merge "Add posix_memalign to API16's libc.so, and to stdlib.h for API 16, 17, 18, 19"