doc: Add zephyr/include to the Doxygen input

The Memory Allocation (alloc_api), PM Runtime (pm_runtime) and DMA
(sof_dma_drivers, sof_dma_copy_func) API groups stopped appearing in the
generated documentation when their headers were decoupled into
zephyr/include during the RTOS split. The Doxygen INPUT still listed only
src/include and xtos/include, so those Zephyr-era headers were no longer
scanned and sof-docs reported "Cannot find group" for all four.

Add zephyr/include to the INPUT. These headers declare exactly those four
groups and no others, with no group-name overlap with xtos/include, so
this recovers the missing API docs without introducing duplicates.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>

Author: lrgirdwo

doc/sof.doxygen.in

@@ -10,7 +10,8 @@ INPUT            = @top_srcdir@/src/include/ipc \
                    @top_srcdir@/src/include/kernel \
 		   @top_srcdir@/src/include/user \
                    @top_srcdir@/src/include/sof \
-                   @top_srcdir@/xtos/include
+                   @top_srcdir@/xtos/include \
+                   @top_srcdir@/zephyr/include
 # Exlude some 3rd party codec headers with external references
 EXCLUDE		 = @top_srcdir@/src/include/sof/audio/MaxxEffect
 RECURSIVE	 = YES