shell: Use AM_CPPFLAGS rather than INCLUDES

This is deprecated in newer automake versions, and this causes warnings
with automake 1.14:
panels/printers/Makefile.am:3: warning: 'INCLUDES' is the old name for
AM_CPPFLAGS' (or '*_CPPFLAGS')

https://bugzilla.gnome.org/show_bug.cgi?id=732189

shell/Makefile.am

@@ -1,4 +1,5 @@
-INCLUDES =					\
+AM_CPPFLAGS =					\
+	-DGNOMELOCALEDIR="\"$(datadir)/locale\""\
 	-I$(top_srcdir)				\
 	$(SHELL_CFLAGS)				\
 	$(CHEESE_CFLAGS)			\
@@ -87,8 +88,6 @@ if BUILD_BLUETOOTH
 gnome_control_center_LDADD += $(top_builddir)/panels/bluetooth/libbluetooth.la
 endif
 
-AM_CPPFLAGS = -DGNOMELOCALEDIR="\"$(datadir)/locale\""
-
 # Dbus service file
 servicefiledir = $(datadir)/dbus-1/services
 servicefile_in_files = org.gnome.ControlCenter.service.in