ANDROID: staging: android: ion: Fix compilation error for ion_trace.h

Mark __ion_ptr_to_hash as __maybe_unused to fix compilation errors
in environments where the -Wunused-function compilation flag is
used, and tracepoints are not enabled.

Bug: 162007514
Fixes: 0f73cdae ("ANDROID: add ion_stat tracepoint to common kernel")
Change-Id: I2eee59d137bb47b40c538f6e2abc60d9f08ca6cd
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>