src: Fix missing ibusenumtypes.h with parallel build

When `./autogen.sh && make -C src ibusresources.c -j2` runs,
it causes a build failure due to no ibusenumtypes.h file.

The chained dependencies are:
ibusresources.c -> compose/sequences-$(ENDIAN)-endian
-> gen-internal-compose-table -> gencomposetable.c

gencomposetable.c includes ibus.h and it causes no ibusenumtypes.h with
the parallel build with multi CPUs.

ibusenumtypes.h is in BUILT_SOURCES so it is built (with other files in
BUILT_SOURCES) before any other rules.
But ibus.h is included by gencomposetable.c which is needed to generate
ibusresources.c in BUILT_SOURCES.

Since gencomposetable.c is an internal build source file and
it doesn't have to include ibus.h but can include each ibus header
files.
Deleting ibus.h in gencomposetable.c can fix the build failure.

BUG=#2523

src/gencomposetable.c

@@ -20,7 +20,7 @@
  * USA
  */
 
-#include <ibus.h>
+/* #2523 Should not include <ibus.h> but each IBus header file. */
 #include <glib.h>
 #include <glib/gstdio.h>
 #include <errno.h>