trusty: add warn_unused_result for mmap() & munmap() am: 277c60c09b am: 02e860c998

Original change: https://android-review.googlesource.com/c/trusty/external/musl/+/3256820

Change-Id: I65a2b1550936dc49b1838c5bb1bf8ba209bfb984
Signed-off-by: Automerger Merge Worker <[email protected]>