core: Fix include path to use top_builddir as well for generated headers

With the new generated headers for the CRC code, we need to include
the builddir as well in case we're building out of tree.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/src/Makefile.am b/src/Makefile.am
index 89b8138..6c0398b 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -4,7 +4,7 @@
 # Please read Chapter 6 "Library interface versions" of the libtool documentation before making any modification
 LIBVERSION=3:0:0
 
-INCLUDES = $(all_includes) -I$(top_srcdir)/include
+INCLUDES = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)/include
 AM_CFLAGS = -fPIC -Wall
 
 lib_LTLIBRARIES = libosmocore.la