keyboard: 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

panels/keyboard/Makefile.am

@@ -52,7 +52,7 @@ xml_DATA     = $(xml_in_files:.xml.in=.xml)
 pkgconfigdir = $(datadir)/pkgconfig
 pkgconfig_DATA = gnome-keybindings.pc
 
-INCLUDES = \
+AM_CPPFLAGS = \
 	-DGNOMELOCALEDIR="\"$(datadir)/locale\""
 
 CLEANFILES = \