Rename 'statistics.c' to 'counter.c'

With stat_item, stats.c and stats_statsd.c, it is becoming a bit
difficult to understand file naming.  Also, the 'statistics.c' file
actually only contained osmo_counter handling, so let's rename it to
counter.c altogether.

Change-Id: I2cfb2310543902b7da46cb15a76e2da317eaed7d
diff --git a/src/Makefile.am b/src/Makefile.am
index 8e7ef4b..4925eed 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -14,7 +14,7 @@
 
 libosmocore_la_LIBADD = $(BACKTRACE_LIB) $(TALLOC_LIBS)
 libosmocore_la_SOURCES = timer.c timer_gettimeofday.c select.c signal.c msgb.c bits.c \
-			 bitvec.c bitcomp.c statistics.c fsm.c \
+			 bitvec.c bitcomp.c counter.c fsm.c \
 			 write_queue.c utils.c socket.c \
 			 logging.c logging_syslog.c logging_gsmtap.c rate_ctr.c \
 			 gsmtap_util.c crc16.c panic.c backtrace.c \