BUG/MINOR: trace: fix hardcoded level for TRACE_PRINTF

Level argument was not ignored by TRACE_PRINTF due to an hardcoded value
of TRACE_LEVEL_DEVELOPER inside the macro.

This must be backported up to 2.6.
diff --git a/include/haproxy/trace.h b/include/haproxy/trace.h
index bf9885d..ad69957 100644
--- a/include/haproxy/trace.h
+++ b/include/haproxy/trace.h
@@ -134,7 +134,7 @@
 			_msg_len = snprintf(_msg, sizeof(_msg), (fmt), ##args);	\
 			if (_msg_len >= sizeof(_msg))				\
 				_msg_len = sizeof(_msg) - 1;			\
-			_trace(TRACE_LEVEL_DEVELOPER, (mask), TRACE_SOURCE,	\
+			_trace((level), (mask), TRACE_SOURCE,	\
 			       trc_loc, func, a1, a2, a3, a4,			\
 			       &trace_no_cb, ist2(_msg, _msg_len));		\
 		}								\