rust: Fix absolute path OUT_DIR bug

The OUT_DIR for rustc is incorrectly calculated when Soong's OUT_DIR
is set to an absolute path through OUT_DIR_COMMON_BASE. This breaks
compilation when rustc is unable to find files that have beein included
through the include! macro.

Bug: 172952634
Test: OUT_DIR_COMMON_BASE=/some/abs/path m
Change-Id: Id447630a7774c1dec655f65dd227c144a159e1b1