ANDROID: ignore compiler tag __must_check for GENKSYMS

GENKSYMS shouldn't care about the __must_check compiler flag. So strip
it out for GENKSYMS.

Signed-off-by: Will McVicker <willmcvicker@google.com>
Bug: 153478475
Test: compile, check crc
Change-Id: I512639a4f719037728ffbfa48e7b766510c7d726