MINOR: trace: make trace sources read_mostly

The trace sources are checked at plenty of places in the code and their
contents only change when trace status changes, let's mark them read_mostly.
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index ddb2a60..f938a57 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -335,7 +335,7 @@
 	{ /* end */ }
 };
 
-static struct trace_source trace_fcgi = {
+static struct trace_source trace_fcgi __read_mostly = {
 	.name = IST("fcgi"),
 	.desc = "FastCGI multiplexer",
 	.arg_def = TRC_ARG1_CONN,  // TRACE()'s first argument is always a connection
diff --git a/src/mux_h1.c b/src/mux_h1.c
index 5b23a29..aa885b8 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -244,7 +244,7 @@
 	{ /* end */ }
 };
 
-static struct trace_source trace_h1 = {
+static struct trace_source trace_h1 __read_mostly = {
 	.name = IST("h1"),
 	.desc = "HTTP/1 multiplexer",
 	.arg_def = TRC_ARG1_CONN,  // TRACE()'s first argument is always a connection
diff --git a/src/mux_h2.c b/src/mux_h2.c
index a42d573..7d2c64e 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -372,7 +372,7 @@
 	{ /* end */ }
 };
 
-static struct trace_source trace_h2 = {
+static struct trace_source trace_h2 __read_mostly = {
 	.name = IST("h2"),
 	.desc = "HTTP/2 multiplexer",
 	.arg_def = TRC_ARG1_CONN,  // TRACE()'s first argument is always a connection
diff --git a/src/mux_pt.c b/src/mux_pt.c
index 3a36f37..86b86e2 100644
--- a/src/mux_pt.c
+++ b/src/mux_pt.c
@@ -76,7 +76,7 @@
 	{ /* end */ }
 };
 
-static struct trace_source trace_pt = {
+static struct trace_source trace_pt __read_mostly = {
 	.name = IST("pt"),
 	.desc = "Passthrough multiplexer",
 	.arg_def = TRC_ARG1_CONN,  // TRACE()'s first argument is always a connection