cpu/native: malloc.h for osx: fix doxygen, move

cpu/native/Makefile.include

@@ -1,8 +1,8 @@
 export NATIVEINCLUDES += -I$(RIOTCPU)/native/include -I$(RIOTBASE)/sys/include
 
-# Local include for malloc.h on OSX
+# Local include for OSX
 ifeq ($(BUILDOSXNATIVE),1)
-    export NATIVEINCLUDES += -I$(RIOTBASE)/sys/malloc/include
+    export NATIVEINCLUDES += -I$(RIOTCPU)/native/osx-libc-extra
 endif
 
 export USEMODULE += periph

cpu/native/osx-libc-extra/malloc.h

@@ -7,7 +7,7 @@
  */
 
 /**
- * @ingroup     sys
+ * @ingroup     native_cpu
  *
  * @brief       Malloc header for use with native on OSX since there is no
  *              malloc.h file in the standard include path.