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/include/Makefile.am b/include/Makefile.am
index 4e92d55..4714003 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -12,6 +12,7 @@
                        osmocom/core/bitcomp.h \
                        osmocom/core/byteswap.h \
                        osmocom/core/conv.h \
+                       osmocom/core/counter.h \
                        osmocom/core/crc16.h \
                        osmocom/core/crc16gen.h \
                        osmocom/core/crc32gen.h \