Clean up Makefile.win32 "-I" flag additions.

The PGXS-case directory does not exist in the non-PGXS case, and vice
versa.  Add one or the other, not both.  This is essentially cosmetic.
It makes Makefile.win32 more like the similar Makefile.global code.

src/makefiles/Makefile.win32

 # src/makefiles/Makefile.win32
 
-# Use replacement include files for those missing on Win32
-override CPPFLAGS+="-I$(top_srcdir)/src/include/port/win32"
-
 ifdef PGXS
 BE_DLLLIBS= -L$(libdir) -lpostgres
+override CPPFLAGS+= -I$(includedir_server)/port/win32
 else
 BE_DLLLIBS= -L$(top_builddir)/src/backend -lpostgres
+override CPPFLAGS+="-I$(top_srcdir)/src/include/port/win32"
 endif
 
 AROPT = crs
@@ -44,11 +43,6 @@ ifneq (,$(findstring src/pl/plpython,$(subdir)))
 override CPPFLAGS+= -DUSE_DL_IMPORT
 endif
 
-# special win32 headers are provided here
-ifdef PGXS
-override CPPFLAGS+= -I$(includedir_server)/port/win32
-endif
-
 # it is better to install shared-libraries anyway?
 # may be overriden with make MAKE_DLL=false install
 ifndef MAKE_DLL