Make headerfile extaccess.h accessible to pxf extension

[hpandeycodeit]

PXF external-table is dependent on fileam.h. In GP7, this file is renamed to extaccess.h and when we install the gpdb, the header file is not copied to $GPHOME/include/postgresql/server/extension/gp_exttable_fdw.

By adding this HEADERS flag in the Makefile, the header file will get copied to the gp_exttable_fdw extension and can be referenced in the PXF code.