MAINTAINERS: Add memory_mapping.h and memory_mapping.c to "Memory API"

Both files logically belong to "Memory API" and are not yet listed
anywhere else explicitly. Let's add them to "Memory API".

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210723100532.27353-4-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

MAINTAINERS

@@ -2460,9 +2460,11 @@ F: include/exec/memop.h
 F: include/exec/memory.h
 F: include/exec/ram_addr.h
 F: include/exec/ramblock.h
+F: include/sysemu/memory_mapping.h
 F: softmmu/dma-helpers.c
 F: softmmu/ioport.c
 F: softmmu/memory.c
+F: softmmu/memory_mapping.c
 F: softmmu/physmem.c
 F: include/exec/memory-internal.h
 F: scripts/coccinelle/memory-region-housekeeping.cocci