logging: include stdarg.h for va_list

fixes tiny compile error:

  CC     socket.lo
  In file included from socket.c:13:
  ../include/osmocom/core/logging.h:31: error: expected declaration
specifiers or ‘...’ before ‘va_list’
  make[3]: *** [socket.lo] Error 1

include/osmocom/core/logging.h

@@ -9,6 +9,7 @@
 
 #include <stdio.h>
 #include <stdint.h>
+#include <stdarg.h>
 #include <osmocom/core/linuxlist.h>
 
 /*! \brief Maximum number of logging contexts */