Fix failure to build gramparse.h standalone in vpath builds

Add include directory in a similar fashion as 829906fb6c.

Per buildfarm animal crake

src/tools/pginclude/cpluspluscheck

@@ -171,6 +171,8 @@ do
 		EXTRAINCLUDES="$python_includespec" ;;
 	    src/interfaces/ecpg/*)
 		EXTRAINCLUDES="-I $builddir/src/interfaces/ecpg/include -I $srcdir/src/interfaces/ecpg/include" ;;
+		src/backend/parser/*)
+		EXTRAINCLUDES="-I $builddir/src/backend/parser/" ;;
 		src/backend/utils/adt/*)
 		EXTRAINCLUDES="-I $builddir/src/backend/utils/adt/" ;;
 	    *)

src/tools/pginclude/headerscheck

@@ -154,6 +154,8 @@ do
 		EXTRAINCLUDES="$python_includespec" ;;
 	    src/interfaces/ecpg/*)
 		EXTRAINCLUDES="-I $builddir/src/interfaces/ecpg/include -I $srcdir/src/interfaces/ecpg/include" ;;
+		src/backend/parser/*)
+		EXTRAINCLUDES="-I $builddir/src/backend/parser/" ;;
 		src/backend/utils/adt/*)
 		EXTRAINCLUDES="-I $builddir/src/backend/utils/adt/" ;;
 	    *)