build: Replace deprecated INCLUDES with AM_CPPFLAGS

Thanks to mnhauke

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/src/Makefile.am b/src/Makefile.am
index 922013f..95a9d6b 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -1,4 +1,4 @@
-INCLUDES = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)/
+AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)/
 AM_CFLAGS = -Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOVTY_CFLAGS) $(PCAP_CFLAGS)
 
 bin_PROGRAMS = osmo_pcap_client osmo_pcap_server