Add statistics subdirectory to Makefile.

Commit 7b504eb282 overlooked this.

Report and patch by Kyotaro Horiguchi

Discussion: http://postgr.es/m/20170608.145852.54673832.horiguchi.kyotaro@lab.ntt.co.jp

src/include/Makefile

@@ -20,7 +20,7 @@ all: pg_config.h pg_config_ext.h pg_config_os.h
 SUBDIRS = access bootstrap catalog commands common datatype \
 	executor fe_utils foreign \
 	lib libpq mb nodes optimizer parser postmaster regex replication \
-	rewrite storage tcop snowball snowball/libstemmer tsearch \
+	rewrite statistics storage tcop snowball snowball/libstemmer tsearch \
 	tsearch/dicts utils port port/atomics port/win32 port/win32_msvc \
 	port/win32_msvc/sys port/win32/arpa port/win32/netinet \
 	port/win32/sys portability