busdma_bounce: fix unused-function warning on powerpc

static inline, and powerpc's own must_bounce() never
calls it.

Reviewed by:	adrian
Differential Revision:	https://reviews.freebsd.org/D58825

sys/kern/subr_busdma_bounce.c

@@ -151,7 +151,7 @@ busdma_sysctl_tree_top(struct bounce_zone *bz)
  * Returns true if the address falls within the tag's exclusion window, or
  * fails to meet its alignment requirements.
  */
-static bool
+static inline bool
 addr_needs_bounce(bus_dma_tag_t dmat, bus_addr_t paddr)
 {