Add exclusion to headercheck

src/include/common/unicode_combining_table.h is currently not meant to
be included standalone.  Things could be refactored to allow it, but
that would be beyond the present purpose.  So adding an exclusion here
seems best.

Discussion: https://www.postgresql.org/message-id/10754.1579535012@sss.pgh.pa.us

src/tools/pginclude/cpluspluscheck

@@ -110,6 +110,9 @@ do
 	test "$f" = src/pl/plpython/spiexceptions.h && continue
 	test "$f" = src/pl/tcl/pltclerrcodes.h && continue
 
+	# Also not meant to be included standalone.
+	test "$f" = src/include/common/unicode_combining_table.h && continue
+
 	# We can't make these Bison output files compilable standalone
 	# without using "%code require", which old Bison versions lack.
 	# parser/gram.h will be included by parser/gramparse.h anyway.

src/tools/pginclude/headerscheck

@@ -106,6 +106,9 @@ do
 	test "$f" = src/pl/plpython/spiexceptions.h && continue
 	test "$f" = src/pl/tcl/pltclerrcodes.h && continue
 
+	# Also not meant to be included standalone.
+	test "$f" = src/include/common/unicode_combining_table.h && continue
+
 	# We can't make these Bison output files compilable standalone
 	# without using "%code require", which old Bison versions lack.
 	# parser/gram.h will be included by parser/gramparse.h anyway.