libosmogapk: rename the 'logging.c' to 'common.c'

There are not so much code, related to internal logging subsystem.
So, there is no reason to keep a few lines in a dedicated file.
In the future one may also be used for other routines.

src/Makefile.am

@@ -76,9 +76,9 @@ libosmogapk_la_SOURCES += \
 	benchmark.c \
 	$(NULL)
 
-# Logging
+# Common routines
 libosmogapk_la_SOURCES += \
-	logging.c \
+	common.c \
 	$(NULL)
 
 # libosmogapk representative application

src/common.c

@@ -17,6 +17,7 @@
  * along with gapk.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+/* Internal GAPK logging */
 int osmo_gapk_log_init_complete = 0;
 int osmo_gapk_log_subsys;