Adding libpq in Contrib module

To include libpq functions in the contrib module the following header should be included:

#include "libpq-fe.h"

However, it throws the following error:

error: libpq-fe.h: No such file or directory

This can be solved by including the following in Makefile of that contib

PG_CPPFLAGS = -I$(libpq_srcdir)

This includes the option -I../../src/interfaces/libpq during make and hence the header file becomes accessible.