Create additional subdirectory for libcurl headers so that curl headers can be included the conventional way

src/bnetd/account_email_verification.cpp

@@ -22,7 +22,7 @@
 #include <random>
 #include <string>
 
-#include <curl.h>
+#include <curl/curl.h>
 
 #include "common/eventlog.h"
 

src/bnetd/smtp.cpp

@@ -19,7 +19,7 @@
 #include <ctime>
 #include <string>
 
-#include <curl.h>
+#include <curl/curl.h>
 #include <fmt/core.h>
 #include <fmt/chrono.h>