tflite: Add libm in linker options

BUG=b:305999697
TEST=Add async_delegate_test into ebuild then emerge tensorflow

Change-Id: Iab6ed2d49a583211f41f5541887273952b90ebff
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/tflite/+/5380175
Commit-Queue: Tommy Chiang <ototot@google.com>
Reviewed-by: Tommy Chiang <ototot@google.com>
Auto-Submit: Shik Chen <shik@chromium.org>
Tested-by: Shik Chen <shik@chromium.org>