gtphub: cosmetic: rename a file.

gtphub_ext.c's initial purpose was to wrap a specific function. The file
then turned into everything related to DNS, which fits pretty well. Rename
to gtphub_ares.c.

Tweak the header comment to reflect the new file name.

Sponsored-by: On-Waves ehi
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index 5e115eb..634948d 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -34,7 +34,7 @@
 			-lgtp $(OSMO_LIBS) $(LIBOSMOABIS_LIBS) $(LIBCARES_LIBS) \
 			$(LIBCRYPTO_LIBS) -lrt
 
-osmo_gtphub_SOURCES =	gtphub_main.c gtphub.c gtphub_ext.c gtphub_vty.c \
+osmo_gtphub_SOURCES =	gtphub_main.c gtphub.c gtphub_ares.c gtphub_vty.c \
 			sgsn_ares.c gprs_utils.c
 osmo_gtphub_LDADD = 	\
 			$(top_builddir)/src/libcommon/libcommon.a \