Added a ring buffer log target to store the last N log messages.

The log target can be used via log alarms and show alarms.
Why? This feature was proposed/requested at
http://openbsc.osmocom.org/trac/wiki/Tasks/ErrorLogTarget
All messages use the same amount of space, prioritizing simplicity.
diff --git a/include/Makefile.am b/include/Makefile.am
index 60b9ea9..317968a 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -16,6 +16,7 @@
                        osmocom/core/linuxlist.h \
                        osmocom/core/linuxrbtree.h \
                        osmocom/core/logging.h \
+                       osmocom/core/loggingrb.h \
                        osmocom/core/msgb.h \
                        osmocom/core/panic.h \
                        osmocom/core/prim.h \
@@ -25,6 +26,7 @@
                        osmocom/core/signal.h \
                        osmocom/core/socket.h \
                        osmocom/core/statistics.h \
+                       osmocom/core/strrb.h \
                        osmocom/core/timer.h \
                        osmocom/core/utils.h \
                        osmocom/core/write_queue.h \