Revert change of ecpglib major version

The major version of ecpglib was changed in
bd7c95f0, apparently without
justification.  Revert this, since nothing has changed in this library
except some added functions.

Discussion: https://www.postgresql.org/message-id/flat/48ee4c56-e1df-b39d-2cad-c7d80b120eb5%402ndquadrant.com

src/interfaces/ecpg/ecpglib/Makefile

@@ -15,7 +15,7 @@ include $(top_builddir)/src/Makefile.global
 
 PGFILEDESC = "ECPG - embedded SQL in C"
 NAME= ecpg
-SO_MAJOR_VERSION= 7
+SO_MAJOR_VERSION= 6
 SO_MINOR_VERSION= $(MAJORVERSION)
 
 override CPPFLAGS := -I../include -I$(top_srcdir)/src/interfaces/ecpg/include \