build: remove unused all_includes and use AM_CPPFLAGS

Preprocessor flags are best placed in AM_CPPFLAGS. Remove use of the
unused all_includes variable, which is never set.
diff --git a/utils/Makefile.am b/utils/Makefile.am
index 6a3cd12..51c884a 100644
--- a/utils/Makefile.am
+++ b/utils/Makefile.am
@@ -1,5 +1,6 @@
 if ENABLE_UTILITIES
-AM_CFLAGS = -Wall $(all_includes) -I$(top_srcdir)/include
+AM_CPPFLAGS = -I$(top_srcdir)/include
+AM_CFLAGS = -Wall
 
 bin_PROGRAMS = osmo-arfcn osmo-auc-gen