Patch reverts upstream commit:
https://github.com/collectd/collectd/commit/6124da7a48f28f54fc09ebeb942d1037516fe6ab

The commit changed the detection path due to FreeBSD issues,
but apparently affects also Openwrt buildroot negatively.

Original explanation:
  From 6124da7a48f28f54fc09ebeb942d1037516fe6ab Mon Sep 17 00:00:00 2001
  Subject: [PATCH] Fix libmodbus detection on FreeBSD

  We look for modbus/modbus.h in /usr/local/include/modbus
  but we should look for modbus.h

  This is only an issue on FreeBSD since /usr/local/include is not
  in the default search path.

Reversed patch to be applied:

--- a/configure.ac
+++ b/configure.ac
@@ -3278,7 +3278,7 @@ then
 	SAVE_CPPFLAGS="$CPPFLAGS"
 	CPPFLAGS="$CPPFLAGS $with_libmodbus_cflags"
 
-	AC_CHECK_HEADERS(modbus.h, [], [with_libmodbus="no (modbus.h not found)"])
+	AC_CHECK_HEADERS(modbus/modbus.h, [], [with_libmodbus="no (modbus/modbus.h not found)"])
 
 	CPPFLAGS="$SAVE_CPPFLAGS"
 fi
--- a/src/modbus.c
+++ b/src/modbus.c
@@ -26,7 +26,7 @@
 #include "configfile.h"
 #include "plugin.h"
 
-#include <modbus.h>
+#include <modbus/modbus.h>
 #include <netdb.h>
 #include <sys/socket.h>