Replace INCLUDES with AM_CPPFLAGS

Automake 1.13 complains that INCLUDES is obsoleted now.

Makefile.am

@@ -5,7 +5,8 @@ lib_LTLIBRARIES = libopus.la
 
 DIST_SUBDIRS = doc
 
-INCLUDES = -I$(top_srcdir)/include -I$(top_srcdir)/celt -I$(top_srcdir)/silk -I$(top_srcdir)/silk/float -I$(top_srcdir)/silk/fixed
+AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/celt -I$(top_srcdir)/silk \
+              -I$(top_srcdir)/silk/float -I$(top_srcdir)/silk/fixed
 
 include celt_sources.mk
 include silk_sources.mk