Fix suggested layout for PGXS makefile

Custom rules must come after pgxs inclusion, not before, because any
rule added before pgxs will break the default 'all' target.

Author: Cédric Villemain <cedric@2ndquadrant.fr>

src/makefiles/pgxs.mk

@@ -9,12 +9,13 @@
 # Use the following layout for your Makefile:
 #
 #   [variable assignments, see below]
-#   [custom rules, rarely necessary]
 #
 #   PG_CONFIG = pg_config
 #   PGXS := $(shell $(PG_CONFIG) --pgxs)
 #   include $(PGXS)
 #
+#   [custom rules, rarely necessary]
+#
 # Set one of these three variables to specify what is built:
 #
 #   MODULES -- list of shared-library objects to be built from source files